Fermi's Paradox, Explained with Science Fiction

 

In 1940, the Italian physicist Enrico Fermi proposed a question that has captivated both scientists and science fiction writers. Why is it so hard to find aliens? By every logical measure, they should be all over the universe, but no matter what we do, we can't find anything definitively proving the existence of life on other planets. This conundrum - the apparent contradiction between the probability of aliens existing and our inability to find evidence of their existence - has become known as Fermi's Paradox. 

Enrico Fermi, the man who popularized the question of why we can't find extra-terrestrial life.

Half a century later, we still don't have a definitive answer, although scientists have come up with a variety of plausible explanations. Despite our attempts to search for them, aliens still remain confined to the worlds of science fiction. Why is that? Well, that's where things get interesting.

Introduction: Paradoxical Reasoning

In our galaxy alone, there are 100 billion stars of varying size, brightness and temperature, each of which has planets in their orbit. The probability that at least some of those stars have planets with favourable conditions for life is, to put it bluntly, astronomical. And given the universe is around 13.8 billion years old, there has easily been more than enough time for life to not only evolve, but to become an intelligent civilization capable of space travel. 

By this reasoning, there should be aliens everywhere. So where are they? Why, despite our best efforts, can't we find a shred of proof that alien life exists? Not even so much as an alien bacterium. No signals, no specimens, no (credible) observations. Not even a fossil. SETI's been trying for decades and they've still got nothing. None of the many probes we've sent out, including the ones specifically designed for finding alien life, have ever turned up anything.

Even the few leads we have keep failing to provide anything concrete. None of our probes have found anything defintively answering whether there is life on Mars. And until we can get a probe out to one of the Jovian moons to actually begin investigating, we can't get a clear answer on whether Europa, Enceladus, or Callisto have anything in their liquid oceans. 

The "wow" signal of 1977 seemed promising, but the technical limitations of the Big Ear telescope  make it hard to verify for sure. The discovery of "Tabby's Star" fueled speculation about an alien megastructure due to its unusual fluctuation in brightness, but this hypothesis was quickly met with skepticism by scientists. 

In 1977, the Big Ear radio telescope picked up an anomalous signal from Sagitarius, possibly alien in origin. However, it's technical limitations prevented it from being conclusively identified.

While we don't have a clear answer, there are several interesting hypotheses that have been offered as potential solutions. 

The Great Filter

One of the most popular ideas explaining Fermi's Paradox proposes that every species faces a challenge which is exceedingly difficult, if not downright impossible, to overcome. This hypothesis has become known as the "Great Filter." Sooner or later, they will encounter this barrier, and from there, it would be simple Darwinian evolution. Any species that can adapt to this problem will survive, while any that can't are "filtered out."

Exactly what this filter is... that's a matter of debate. We can't even be sure if we've already passed it or if it's still coming. And that's, of course, assuming there's only one. If this hypothesis is true, there could be hundreds or thousands of potential filters affecting the evolution of life. Either way, the basic principle is the same. The filter, at the very least, reduces the number of species that evolve past it, if not wiping them out entirely. 

The entire premise of Dead Space offers a pretty good example of one such threshold. The game's setup involves a future where humanity has used up its available resources and had to start finding other ways to produce energy. It's later revealed that this is actually a common evolutionary pitfall. Here, we see a proposal for a form the great filter could take- every time an intelligent civilization develops, they inevitably reach a point where they grow beyond their ability to sustain themselves. 

The setup of Dead Space involves a future where humanity has grown to a point where it struggles to sustain itself, leading to the extreme practice of planet cracking for resources.

The games even portray this as a big enough filter that it can actually be exploited. The entire life cycle of the series' main antagonists, the Brethren Moons, literally depends on other species getting caught in the great filter. Because when a spacefaring civilization struggles to find the resources to keep itself going, a seemingly infinite and easily replicable power source like the markers becomes very appealing. 

The Dark Forest

The universe of Warhammer 40k has no shortage of things to be afraid of. Whether it's any of the other factions, inhospitable planets, spatial anomalies, ancient weapons of mass destruction, the incoming swarm of tyranids bent on consuming all biomass in the galaxy, the chaos gods who delight in messing with everyone, the possibility of accidentally creating more chaos gods just by feeling too much emotion, and a myriad of possible fates worse than death, there is a lot to be frightened by in the cosmos. This makes it a good demonstration of the rationale behind the dark forest hypothesis.

Put simply, the dark forest offers a very bleak answer to Fermi's paradox. It's not that alien life doesn't exist, it's because they're hiding out of self-preservation. This theory compares the cosmos to a forest populated by a variety of creatures, but each one deals with various threats that force them to hide to survive. Revealing themselves makes them vulnerable to potential threats.

Applying this rationale to a cosmic scale, the dark forest would argue that the galaxy is full of life, but it prefers to remain hidden to avoid drawing attention from potential threats. That fear may or may not be justified, but contact isn't worth the risk. This hypothesis also offers an even bleaker outlook as it calls into question whether we should even be making efforts to locate extraterrestrial life. If there is something to be afraid of, is SETI nothing more than us recklessly endangering ourselves and drawing attention from something we really don't want to notice us?

If the Dark Forest hypothesis is to be believed, than Eleanor Arroway is actively endangering humanity by risking the attention of bigger threats.

Warhammer 40k may be a bit more on the extreme end, but it is a galaxy full of things most species would want to hide from, including each other. If you knew about the dangers posed by tyranids, drukhari pirates, and orks, you'd do well to hide from them if you could - especially if you lack the means to fight back. Sometimes you even have good reason to be afraid of your own faction - the Imperium isn't exactly above purging its own worlds and some space marine chapters really don't care about civillian collateral damage. In a world like that, is it really any wonder that a lot of civilizations would want to remain hidden?

If you knew these things could come to your planet with the goal of eating anything, why wouldn't you try to hide?

The Dark Forest hypothesis suggests that alien civilizations are simply reluctant to reach out because they don't know the potential consequences. If we did make contact, how would we know we weren't opening the door to something really bad? What if we accidentally reached a brutal alien empire bent on conquering the galaxy and in doing so caused our own subjugation? What if we accidentally attracted a tyranid-like swarm that wanted to consume all life on Earth? And even if we didn't, how could we be sure that such dangers aren't out there somewhere.

Different Forms of Life

One of the more creative, yet quite plausible solutions to Fermi's Paradox is that we've simply failed to recognize life on other planets. Think about how aliens are often portrayed in science fiction media. 

Aliens are often portrayed as humanoid. On the cheaper end, it might just be humans with weird costumes or hairstyles. Higher production values can allow for some more creative options, like unusual skin tones, unique facial features, extra limbs, bits and pieces of other creatures, or different heights. Sometimes, the writers might add to their alien qualities by giving them strange anatomy. But either way, it still comes down to using the human body as a baseline. 

We see this all the time - Star Wars, Star Trek, Doctor Who, Marvel, Warhammer 40,000, Farscape, Mass Effect, Hitchhiker's Guide to the Galaxy, Avatar. All of them feature aliens modeled on the human body. Even the stereotypical cartoon alien - the green/grey figure with a big head and large black eyes, still uses a bipedal form, albeit one with its proportions altered.

And it doesn't take a genius to figure out why this is the case. It's a practical decision. Since we don't have genuine alien actors willing to appear in our movies and TV shows, we have to settle for casting humans in the role. And while makeup prosthetics can go a long way in the right hands, they still have to physically accommodate a human actor.

Sci-fi aliens often appear humanoid so they can be played by humans. If extra-terrestrial life exists, it wouldn't be constrained by such limitations.

Now of course, there are other ways to portray aliens. CGI and puppetry can allow more flexibility when it comes to design, though a CGI model may still require motion capture and a puppet still needs to make room for the puppeteer. And they can be pretty expensive next to putting makeup on a human actor. 

There's also another problem. Our only reference for what life looks like is our own planet. So naturally, when designing aliens, people often draw inspiration from terrestrial organisms. A popular example is to take inspiration from insects and arachnids. Some notable examples include xenomorphs, tyranids, Yanme'e, the "buggers" in Ender's Game, the "arachnids" in Starship Troopers, antlions, at least one variation of the Thing, rachni, zerg, and geonosians. Even LEGO got in on the act with their Insecctoids theme. All of these examples adopt traits from various bugs, with many of them even using a hive mind similar to ants or honeybees.

LEGO's 1998 Insectoids theme followed aliens with a bug motif. A lot of their vehicle designs took inspiration from insect and arachnid features.

Other common ideas include sentient plants or fungi, lizard-people, and wormlike species. All of these follow the same pattern of taking inspiration from life on Earth. But what if that is exactly what's holding us back? Our failure to find any extraterrestrial life, let alone intelligence, is actually because it's so different from anything we would consider "life" that we can't recognize it. 

It makes sense. After all, life that originated independently of Earth would logically have no reason to follow the exact same evolutionary path. Under the right conditions, there could be an extreme form of convergent evolution, where unrelated species develop similar traits due to similar environmental pressures, but it would hardly be an exact copy. There's no reason to think an independently-forming species would even conform to any of our biological classifications. Even the four main kingdoms (plants, animals, fungi, and unicellular life) might go out the window when it comes to alien ecosystems. 

A classic example of such an alien would be H.P. Lovecraft's The Colour out of Space, an organism so peculiar and unlike anything anyone's ever seen that it can only be described as a colour, due to it somehow presenting a colour outside of the visual spectrum. We see the impact the entity leaves on the surrounding environment, how it seems to twist and slowly destroy living things, but we don't actually know what it is. Where did it come from? How does it work? What are its goals, if any? Is it even sentient? Does it act maliciously or purely on instinct? Why does one piece of it go back in the well when the rest of it is leaving? Nobody can find a definitive answer.

An entity that's barely identifiable when it's literally right in front of you and observed in real time would be hard to recognize on another planet, especially with our limited knowledge.

Another great example would be Stanlislaw Lem's novel The Invincible, as well as the video game adaptation of the same name. Both take place on a planet called Regis III, which has a peculiar ecosystem. Life, at least as it is understood by humans, appears in the oceans but is conspicously absent from land. Investigation of the planet reveals it to be a result of a process Lem called "necroevolution," where non-organic matter goes through similar evolutionary pressures to organic life. 

In this case, self-replicating machines competed with local wildlife, and then fought each other over resources until the only ones that remained were the most adaptable. By the time the astronauts find them, they've been incorporated into the planet's ecosystem to the point where sea creatures have evolved to detect electromagnetic fields and avoid the coast. 

Yet for a long time, this stumps the investigators, who struggle to wrap their heads around the notion of metal life forms and how to interact with them. Because the very notion of machines undergoing Darwinian evolution flies in the face of everything understood about biology. Such a phenomenon would be hard to observe, especially when we can't actually visit the planet in question. 

Zoo Hypothesis

Reaching out to extra-terrestrial life comes with two underlying assumptions. First, it assumes they are in fact listening, and they are interested in responding. But what if only half of that was correct. That there were aliens listening to our signals, maybe even studying us? But for whatever reason they prefer not to reply? This is the Zoo Hypothesis. In a zoo, people will observe animals without directly interacting with them. On a broader scale, the Earth is treated like a zoo exhibit observed by aliens.

These aliens could be watching us right now, meticulously recording and documenting human history in the same way that a biologist might observe and document the bahaviour of wild animals. But, for whatever reason, they avoid directly engaging with us. There are plenty of reasons why they might decide not to make themselves known. Maybe they're concerned about the consequences of getting involved, or they see us more as specimens than people. 

But one especially popular idea is they want to ensure humanity evolves on its own, and therefore go out of their way to avoid influencing our cultural development until such a time as we are deemed ready to meet them. For instance, when we reach a specific evolutionary milestone - maybe a big technological breakthrough or some sort of cultural shift.

And by now you might be noticing that this is starting to sound strangely familiar. Because yes, this is literally the foundation of Star Trek's Prime Directive.

Berserker

One of the more bizarre, though not entirely impossible, theories about the lack of aliens is the beserker hypothesis. This one argues the reason we can't find aliens isn't because they don't exist, but rather because there's something out there which keeps wiping them out. This one easily overlaps with the Dark Forest, as it gives an added incentive for other species to remain hidden, and to an extent, it can tie into the Great Filter. 

However, there is a key distinguishing factor here. The Great Filter proposes a common natural barrier or evolutionary pitfall, and the Dark Forest suggests that species try to hide out of fear of each other. The berserker hypothesis proposes the existence of an external agent which is actively locating and destroying life wherever it appears, or at least destroying intelligent civilizations when they reach a certain point.

The culprit in this hypothesis is known as a Von Neuman Probe, a hypothetical self-sustaining and self-replicating machine that locates and destroys life whenever it is detected. Whether it does so maliciously or simply acts on programming instinct, the basic principle is the same. We can't find alien life because there is something which keeps destroying it.

Stanislaw Lem envisioned a smaller-scale version of this concept with the Invincible. Regis III should support life, yet it only does so in the oceans. We eventually learn that an ancient alien race brought self-replicating machines to the planet, machines which ended up competing with local wildlife and ultimately eliminated land-based creatures, leaving the remainder of the planet's life to develop in the oceans, where it was out of reach. 

The microbots featured in The Invincible, referred to by the characters as "fllies" may only occupy one planet, but they provide a good example of just what makes the Berserker hypothesis so terrifying. Not only is there something intentionally preventing the evolution of life (albeit one that's acting on instinct rather than malicious intent), but its ability to self-replicate makes it nearly impossible to eradicate. The ability to make copies, which can make copies which also make copies, means the flies can reproduce faster than we can attack them. Even destroying the planet doesn't guarantee their elimination.

The Reapers of Mass Effect are a great example of berserkers. The whole setup of Mass Effect involves them taking over the galaxy and creating a system where species are harvested every 50,000 years. At the end of each cycle, they arrive, wipe out the dominant species, and use their biomass to produce a new reaper. They are also capable of producing a seemingly endless army to throw at their targets, not helped by an ability to turn captured and fallen enemies into additional soldiers. 

According to Mass Effect, Fermi's Paradox is solved with the knowledge that intelligent civilizations are destroyed by the Reapers.

While the berserker hypothesis often describes a machine, this doesn't neccessarily have to be the case. A similar function could be filled by a living organism. Take, for instance, the Brethren Moons in Dead Space, a species of moon-sized aliens who, like the Reapers, depend on other species' evolution to reproduce. They even have their own berserker-like technology in the form of markers, although these are designed to be copied by others rather than full-on self replicating. The end result is still the same, they systematically destroy the dominant species and use its biomass to create a new moon. 

Warhammer 40k has several examples of organic berserkers. Tyranids and orks, for instance, have survival advantages that make them hard to eradicate. The former can keep building new units with consumed biomass, and the latter can reproduce through spores. Because of their highly adaptable nature, killing tyranids can result in them just coming back stronger. And even if you kill every ork on a planet, you can't keep track of where their spores might land. 

Either one could work for the berserker hypothesis. Why are there no aliens? Maybe because the tyranids keep eating them, or the orks keep destroying or at least wearing them down in constant wars. Even the Imperium, a xenophobic regime that decries alien life, could be seen as an example due to their abundant supply of fanatical humans and eagerness to destroy xenos. 

The big question with this theory is why we haven't found any evidence of these Von Neuman Probes, and there are some plausible explanations for this. Perhaps we have seen them, but lacked the necessary information to recognize what they were. Or they simply haven't found us, or we need to reach a particular evolutionary stage before they take notice. Or maybe, by some strange stroke of luck, we managed to fly under their radar without even realizing it. 

Self Destruction Theory

In many science fiction stories, we often see dangers posed in the form of an external threat. Alien invasions have been a staple of the genre going back to H.G. Wells' The War of the Worlds. But what if the real danger was internal? What if, in fact, an intelligent species could be its own worst enemy? This is self-destruction theory, which posits that we can't find alien life because intelligent civilizations are doomed to be the instruments of their own destruction. 

This certainly tracks with a lot of human history. One doesn't have to dig deep to find examples of us nearly wiping ourselves out, or at least getting set back to the Stone Age. Half the 20th century saw the world trapped in a Cold War as the most powerful countries pointed nuclear bombs at each other, only deterred by the threat of mutually assured destruction. Even now, as we struggle with human-created climate change and keep running into a political deadlock any time anyone offers a solution, it's not out of the question. 

Science fiction has no shortage of hypothetical scenarios in which humans destroy themselves, or at least make enough mistakes to cause a societal collapse. The exact reason might vary, though often it comes as a result of humanity's worst qualities. Capitalism and politics are often attributed to the disaster.

Godzilla was originally created as a metaphor for humanity jumping into nuclear weapons without fully understanding the power they had. The original movie even explained his rampage was a consequence of careless nuclear testing. Even after the Cold War ended, themes of humanity destroying itself through nuclear war have persisted in titles like the Fallout series of games.

Another popular example is the creation of a machine which turns on its creators, often as a result of human carelessness or cruelty. The Terminator films center around an AI which, thanks to human programming, concludes that humans need to be eradicated to fulfill its directive. The apocalypse in Horizon Zero Dawn resulted from businessman Ted Faro investing in self-replicating war machines that could run on biomass and began consuming every living thing on Earth. 

Gears of War is an interesting example. The games center on an ongoing war between the humans and an underground race known as the locusts, each bent on wiping the other out. The irony comes from the implication, and eventual confirmation, that the entire war is humanity's fault and the locusts only exist as a direct consequence of the earlier Pendulum Wars. In short, humans are doomed by their inability to escape their own violent nature.

What if this self-destructive tendency isn't unique to us, but happens sooner ot later to most intelligent species. At some point they reach a step where they inevitably slip up and become their own worst enemy. For instance, by destroying themselves in a nuclear war or building a killer AI.

Take, for instance, the Krell of Forbidden Planet. We learn that they were a highly advanced alien species that built some incredible technology, including machines that could increase a person's intelligence and create from pure imagination. However, as Lieutenant Ostro learns the hard way, this seemingly incredible feat backfired when it also gave physical manifestation to the species' base instincts, resulting in ferocious "monsters from the id" that ultimately wiped out the krell. 

In Forbidden Planet, Dr. Mobius (Walter Pidgeon) demonstrates the advanced technology of the Krell, which included the ability to create from their own imagination. We later learn that this seemingly amazing technology was exactly what caused their downfall.

Warhammer 40k has a more bizarre example with the Aeldari, an alien race that once built a thriving empire across the galaxy. That was until they had too much fun and accidentally created a chaos god of murder, Slaanesh (it's complicated), whose birth alone wiped most of them out, leaving what was left struggling to survive in an already unforgiving universe. While creating a literal god through emotional intensity might be a stretch, at least with our current understanding of the cosmos, it is still a good example of a species destroying itself.

Time and Distance

There has definitely been enough time for life forms to evolve, probably more than enough for some of that to grow into an advanced spacefaring civilization, but there's also been more than enough time for that civilization to fall.

Ancient "precursor" civilizations are a pretty old trope in science fiction. The details may vary, but usually it involves the discovery of ancient ruins and artifacts built by a long-forgotten alien species. They may be good, bad, or neutral. They may be completely extinct or still exist in some barely recognizable form. Sometimes, they even go as far as to claim these aliens visited an ancient Earth or had a hand in humanity's evolution. Whatever the details are, the basic ideas remain the same - this once-thriving civilization existed, met a tragic end, and the consequences of their actions can still be felt today.

We can see this all over science fiction - the Elder-Things in At the Mountains of Madness, the species behind the monoliths in 2001: A Space Odyssey, the Old Ones of Warhammer 40k, the Forerunners in Halo, the Protheans in Mass Effect, the Isu in Assassin's Creed, the Rakata in Star Wars, the Protoss of Starcraft, the engineers of Prometheus, the Krell in Forbidden Planet, the Lyrans in The Invincible, the list goes on. But maybe the idea isn't so far-fetched. 

Perhaps, if we were to find evidence of extra-terrestrial life, this is the most likely way it will happen. Not with first contact, but with us finding what remains. Ruins, artifacts, old technology. Maybe even fossils if they're old enough.

Keep in mind that the universe is currently estimated to be 13 billion years old, and the formation of Earth is actually pretty recent on a cosmic scale. Then there's the issue of distance. Keep in mind that our own galaxy has over 100 billion stars, and it's big enough that light takes 100,000 years to move from side end to the other. And that's not even getting into the even greater distances between galaxies. 

Put simply, maybe the reason we can't find alien life is because it's not all that common for multiple spacefaring civilizations to exist in the same time and place. Contrary to what Star Trek and Mass Effect would suggest, a multi-species organization like Starfleet isn't feasible simply because we can't realistically find other spacefaring species to work with. If intelligent life exists, it could well have evolved in another galaxy too far from us to even see the Milky Way, or maybe we're just a billion years too late for first contact. 

Perhaps, even if there is something out there, finding it just isn't realistic, at least not with any means we currently have. Maybe finding the remains of past civilizations really is the best we can hope for. 

Early Bird

The idea of an alien precursor is certainly an interesting one, but the early bird hypothesis flips the script. What if we are the precursor aliens? The argument here is we can't find any alien civilizations because we're getting into the cosmos early. We are one of, if not in fact the first spacefaring civilization to develop. Other potential species, for whatever reason, simply haven't reached the same technological level as us, and may not until well after humanity is gone.

According to the Early Bird Hypothesis, we're the ones leaving the monoliths and it's future alien civilizations who will find them.

This is an interesting one since it reverses the common trope and offers a scenario not seen in a lot of sci-fi. Imagine in the distant future, some other species figures out space travel, finds Earth, and then explores our ruins. the remnants of the great human civilization. There could be alien archeologists collecting artifacts and trying to piece together human history. 

Rare Earth

Occam's Razor is the principle that, when confronted with multiple possible explanations for something, the simplest is often the most likely. If we apply this reasoning to Fermi's Paradox, it offers a fairly straightforward answer. Maybe the reason we can't find alien life is, in fact, because there's none out there to find. Or at the very least, the formation of living organisms is such a rare occurrence that our odds of actually finding it are almost zero.

Portrayals of Rare Earth have a few variations. They can range from no aliens at all to simply no other intelligent spacefaring species while still allowing alien fauna, to any spacefaring aliens that might have existed being extinct or super uncommon. This can be an interesting approach. Without being able to attribute things to aliens, writers have to get more creative in finding alternative obstacles. A story about humans can focus on human conflicts and human themes. And of course, there's the inevitable existential questions about what it means to be alone in the universe.

Red Dwarf offers a good example of the more extreme view of Rare Earth. Protagonist Dave Lister is accidentally put into stasis for 3 million years, enough time for humanity to go extinct. By the time he emerges, the ship is lost in a cold, empty universe with no signs of alien life. The show deliberately avoids aliens, instead relying on remnants of human creations, spatial anomalies, problems with the ship, conflicts among Dave and his few companions, and his own efforts to avoid going mad from isolation as plot threads.

The plot of Red Dwarf centers around its cast being lost in an empty universe. The closest things to aliens are leftover human creations (robots and genetically modified organisms) and occasionally creatures descended from Earth life.

The Expanse has a somewhat more liberal view, but still takes a similar approach. The show takes place in a future where humans have spread out across the Solar System, but the only trace of any alien life is the mysterious protomolecule. And even when that starts causing problems, it's more about how that fuels human conflict than where it originally came from. We do see alien fauna in later seasons, but that only happens as a result of the ring gate facilitating interstellar travel. 

Combinations

It shoud be noted that while the above theories are all very compelling, they are far from mutually exclusive. We have no way of knowing for sure, which, if any, are correct. But there's no reason to assume that the validity of one discredits the others. They can easily overlap with one another, making it entirely possible that some of them fit together as part of the puzzle, rather than being a whole explanation.

For instance, let's say that the Rare Earth hypothesis is true, and life isn't anywhere near as common as we thought. Could that rarity not be compounded by a Great Filter? And if the berserker hypothesis is true, it would be a good incentive for other species to stay hidden. 

Dead Space, as shown in this very article, actually combines several different possible solutions to Fermi's Paradox:

• Rare Earth: Life of any kind, letalone intelligent life, is already hard to find
• The Great Filter: Every intelligent species that evolves keeps growing to a point where they struggle to support themselves.
• Berserker: The Brethren Moons deliberately and systematically wipe out each spacefaring civilization that emerges. 
• Time and Distance: Humans are unable to make contact because every previous civilization was destroyed before it could last long enough to meet us.
• Zoo: The Brethren Moons intentionally wait until the dominant species reaches the Great Filter before revealing themselves.

Conclusion

We're still no closer to answering the ultimate question at the root of this. Is there alien life out and there and, if so, where is it? The short answer is, we don't know. That's the point of Fermi's Paradox, it challenges us to think about the deeper mysteries of the universe. Maybe all of these hypotheses are correct. Maybe none of them are. Maybe the correct answer, if there is one, hinges on factors we currently have no way of knowing about. Who knows? Maybe someday we'll find some strange piece of evidence that finally points us in the right direction. Maybe we never will.

We've had leads, though so far nothing conclusive. We'd need to find something that definitively, absolutely, beyond the shadow of a doubt proves the existence of a living organism on another planet. Some smoking gun piece of evidence that defies any attempt at natural explanations would be needed. And until such a time as we find that evidence, alien life will have to remain confined to the worlds of science fiction. But through science fiction we can explore these deeper questions about the universe and just what really lies out there. 

Deadliest Alien Parasites, Ranked

One of the greatest examples of nature's cruelty you can find is the existence of parasites. Parasitism is a relationship between two species in which one benefits by harming the other. The one that is being harmed is usually referred to as the host. This is distinct from predation, wherein one species eats another as a food source. A predator will consume what it needs to and then moves on. A parasite will remain tied to its host as long as possible in order to exploit them.

Parasitic organisms can be found in all four biological kingdoms and live in every conceivable biome. They come in many different forms. There are parasitic birds, perhaps most famously the cuckoo, that sneak their eggs into other nests. The Last of Us was inspired by the real-life Cordyceps, a fungus that zombifies insects. Mistletoe survives by stealing nutrients from trees. Anglerfish developed a form of parasitic reproduction where males are permanently fused into the female's body. 

A male anglerfish is permanently infused to a female's body, acting as a parasite by stealing nutrients from anything she eats.
Original photo by Edith A. Widdler

The abundance of parasites has even created its own ecological niche. Several species of fish effectively exploit the existence of parasites by acting as a sort of "cleaning service" that eats them off of other creatures' bodies. 

 

The worst part is it's not even their fault. Parasites don't act maliciously or out of love for causing pain. They do it because natural selection has forced them onto an evolutionary path where they can't survive unless they parasitize. Because parasitism can, unfortunately, be a very effective survival strategy. So if there is alien life somewhere out there in the universe, whether intelligent or not, why shouldn't we expect to see examples of parasitism on other worlds and  in alien ecosystems? 

In honor of spooky season, we're going to dive into the disturbing world of alien parasites. I've collected the grossest, most terrifying parasites I could find from across film, TV, video games, and comics. And we are going to scientifically determine the deadliest parasite science fiction has to offer.

Note: This article was inspired by a piece I previously published on Game Rant. While some of the information from that article was reused, I have rewritten each entry to avoid self-plagiarism (that's apparently a thing) and tried to expand on the original concept, as well as provide updated information when available.

The Imposter- (Among Us)

While Among Us doesn't take itself particularly seriously, it does present a pretty unnerving setup. The cartoon aesthetic, silly cosmetics, and memes might take the edge off your fear, but how would you feel being trapped on a ship with the knowledge that one of the people around you isn't who they claimed to be? 

The Imposter operates by infiltrating spaceships, consuming a crew member, and then assuming their identity. This gives them the ability to blend in, forcing the crew to use deductive reasoning to find them. When played effectively, the imposter can sabotage critical systems and eliminate crew members without drawing suspicion. A particularly crafty imposter can even divert suspicion onto other players, increasing the odds of an innocent person being thrown out the airlock.

Devastating as it is, however, the imposter only works at a small scale, and lacks the spreadability of many parasites. It can eliminate crew members but cannot infect anyone outside of its initial host. This makes it pretty tame in the grand scheme of things.

Fib- (Veggie Tales)

The Fib, also known by the scientific name Fibrilius Minimus, is a parasitic alien that takes a rather unconventional approach to its hosts. Instead of physically infecting them, it feeds off of dishonesty. When it lands in a heavily populated area, Fibrilius Minimus will scout the population in search of an individual who is in some sort of trouble, usually someone who has made a mistake or performed an immoral act and fears the consequences. This person will become their unwitting host. Children are an ideal target, due to their lack of critical thinking skills and ability to come up with lies that easily fall apart under scrutiny.

Upon making contact, the Fib will act as a friend while playing into their host's worries, convincing them it is better to lie about their misdeed than to admit fault. With each lie, Fibrilius Minimus is able to grow larger. Their growth rate accelerates with increased lies, so they will encourage the host to dig themselves deeper by continuing to lie even after being called out. In a matter of hours, Fibrilius Minimus can grow from a tiny ball-shaped alien to a godzilla-sized monster, at which point it will turn on and eat its host. 

Admittedly, we don't have a full picture of the Fib's life cycle, so it's hard to say for sure how easily it spreads. However, individual instances don't seem to be contagious. They tend to focus on a single host and conveniently "disappear" in the presence of witnesses. That said, the final stage does make them capable of destroying a city, so we shouldn't underestimate them. 

However, Fibrilius Minimus's method is also its greatest weakness. Just as it thrives on dishonesty, honesty can bring it down. The only known way to weaken or even kill it is for the original host to come clean about their lies. This is a pretty simplistic solution next to some on the list.

The primary method of travel used by Fibrilius Minimus appears to be via asteroids. The likely way it works is riding on asteroids until they enter the atmosphere of a habitable planet, then ejecting themselves into a populated area. While effective, it is not the most efficient means of spreading as it lacks any control of where it ends up.

Headcrabs (Half-Life)

Anyone familiar with the Half-Life games will be familiar with the infamous headcrabs, a parasitic organism that encases the head of its host. They've faced off against them and witnessed the horrific fates of their victims. The infamous cry of a headcrab zombie betrays the anguish experienced by those unfortunate enough to be hit by one. 

Headcrabs have a simple way of functioning. Once they find a host, they jump up and encase their head (hence the name). They then embed their arms into the host's shoulders and seize control of their nervous system. The process results in the body being contorted and mutated, all while the victim is still alive and fully conscious. The only hope for release is death. It's a cruel fate, one weaponized by the Combine, who love to deal with suspected dissidents by launching a rocket full of headcrabs at them.

However, the very fact that they can be weaponized betrays a much darker truth. As horrific as they may be, headcrabs are actually on the weaker end as far as parasites go. Their only method of infection is to physically secure themself on the host's body, and it is possible (if difficult) to kill one before this happens. With sufficient precautions, they can be handled without issue. The Combine were able to use them easily, vortigaunts are able to consume them as food, and Kleiner even kept one as a pet simply by debeaking it. 

While the results are nightmarish, Headcrabs ultimately lack the spreadability of many others, making them seem tame in comparison to others.

Seed Pods (Invasion of the Body Snatchers)

Seed Pods specialize in infiltration, which isn't unusual for a parasite, but they have one thing others don't- they don't have to physically infect their hosts. All that's needed is for a potential host to be in proximity to a seed pod. The pod produces a copy of the host. When the host falls asleep, they die and get replaced with the copy. That copy will then assume the host's identity while helping to continue spreading seed pods to infect more hosts.

What makes seed pods particularly dangerous is their discretion. They start small and gradually work their way out, carefully blending into the population and diverting any questions or investigation. Often it works so subtly that early signs are easily ignored. They come in the form of things like a kid saying there's something off about his mother, something easily disregarded by others. By the time they make themselves known, they're too late to stop.

Technically it is possible to spot a host- a general lack of emotion is a dead giveaway. However, by the time anyone can start identifying them they will have already spread to a point where they are really hard, if not impossible to avoid. The seed pods themselves can get so widespread that even the most resourceful survivors can be infected purely by chance. It was a total fluke of luck that any government response could be made to them, and that was the ending Don Siegel was forced to put in. His original vision was to end with the protagonist realizing how far it's spread and trying in vain to warn others (and the viewer) about what's coming.

The one limitation the seed pods have is they lack a means of self-spreading. While they can reproduce in alarmingly large numbers, actually spreading requires making use of their hosts and existing technology. Just covering the immediate area requires seed pods to actively be carried or loaded into vehicles to spread. Although this limitation does show them to be resourceful and adaptable to their environment, it does slow them down compared to others.

Genestealers (Warhammer 40k)

Tyranids usually function as predators, rather than parasites. However, there is one exception in one of their more... nefarious strategies. Genestealers are a special kind of Tyranid. They sneak aboard ships to get into worlds populated by other races. Once in place, they begin building up a cult, usually by seeking out outcasts and marginalized groups and slowly working their way into positions of power and influence. Over a few generations, their DNA will also start to infect the local population, producing hybrids. This makes them both a social and a biological parasite.

Left unchecked, a genestealer cult can gain enough power to dismantle a planet's defenses and destroy government infrastructures. Which is exactly the point. The goal, ultimately, is to leave the world vulnerable to a full-on tyranid invasion, essentially using parasitism to aid a predator. They can spread across an entire world without being noticed until it's too late.

The one silver lining, if it can be called that, is that genestealers are usually confined to a single planet. Once the final stage is complete, the genestealer and all its hybrid offspring are consumed as biomass along with everything else, rendering them incapable of spreading further. 

Xenomorphs (Alien)

The titular aliens from Alien function as both parasites and predators. As predators, they are apex hunters. A single fully-grown xenomorph can decimate a group of humans in a matter of hours. A whole colony can be too much even for a group of heavily armed colonial marines. Xenomorphs are great at using their environments to ambush their prey. It doesn't help that they're intelligent and fully capable of strategizing. 

As parasites, on the other hand, they have a messy reproductive cycle involving the infamous and aptly named facehugger and chestburster. The facehugger attaches itself to the host's head and shoves a tube down their throat through which it places the chestburster. Once that's fully grown, the chestburster will violently tear its way out of the host's body, killing them in the process.

As if that weren't bad enough, the facehugger has a cruel trick. It exploits the empathy of others to deter any effort at removal. It tightens its grip on the host's neck if any attempt its made at pulling it off. And the highly corrosive blood prevents it from being cut off without risking grievous harm to the host, the cutter, and any bystanders. Facehuggers can also burn through glass, so wearing a space suit won't offer any protection.

The good news is the facehugger alone isn't contagious. A single facehugger will attach itself to one host and die upon successfully implanting the embryo, so anyone around the host is unlikely to be infected. At least, that's if there's only one of them. While removing the facehugger is a huge gamble at best, it is at least theoretically possible to remove the chestburster via surgery if it is spotted early enough. 

To really get going, xenomorphs need to mass produce eggs... which is exactly what they do. The crew of Nostromo found hundreds of eggs, but were fortunate enough to only see one actually hatch. The settlers at LV-426, fifty-seven years later, weren't so lucky. However, this requires the establishment of a queen and a sufficient supply of potential hosts. While effective, this does take time and requires xenomorphs to jump through a few hoops before they can reach the level of other parasites. 

Zerg (Starcraft)

The Zerg are driven by an endless quest to achieve genetic perfection. To that end, they have an unusual biological quirk- an ability to consume other organisms and absorb their genes. This ability makes them highly adaptable to different environments, including outer space. By taking traits from other species, Zerg can produce a variety of different units for different tasks. There are even zerg who have evolved to act as living spaceships, facilitating interstellar travel. 

Naturally, they spread very easily and can consume entire worlds given the opportunity. While it is possible to resist them, this is easier said than done. They are extremely durable and resistant to most conventional weapons, and their sheer numbers can be overwhelming. There's a reason they are often referred to as a swarm.

Luckily, there is one thing that's stopped them from consuming the galaxy outright. They are pragmatic and can be reasoned with. Under the right circumstances, they can form alliances and make an uneasy truce with other races. And even uphold that truce. They were even open-minded enough to make a human their queen. While certainly not an easy thing to broker, it is more than can be said for a lot of parasites.

Mnggall-Mnggal (Star Wars)

Note: This may or may not be an actual image of Mngall-Mnggnal. It's been alternately labelled as Mgnall-Mnggal and a DND monster named Jubilex, and I have been unable to find the picture's original source, so I can't confirm which it is.

What I said earlier about parasites not being to blame for their actions? Yeah, that all goes out the window with Mnggal-Mnggal- a parasitic creature so twisted and evil, you could probably be forgiven for thinking this was Warhammer 40k's answer to the Thing instead of something out of Star Wars. What makes mnggall-mnggal particularly sick is its malevolence. It doesn't just parasitize to survive; it revels in it. It loves tormenting its hosts and using them to play mind games.

Mnggal-mnggal is essentially a sentient fluid, which makes it easy to infect a potential host. It commonly appears as a seemingly inert pool of black goo, waiting for a potential host. When one appears, it will splash the unfortunate victim, quickly sliding into any opening it can find in the body. A single drop will be more than enough to be lethal. mnggal-mnggalwill begin eating the host's body from the inside out, breaking down organs, tissue, and bone, until only the outer flesh remains. It will then continue to puppet its host until their body inevitably decays. 

This does make mnggal-mnggal easy to spot if one knows where to look. Zombified hosts have several obvious signs. However, what makes mnggal-mnggal dangerous is it's intelligence and desire for cruelty. It won't just infect anything in sight, it will actively communicate with them. mnggal-mnggal will offer lost secrets and forbidden knowledge that it may or may not actually have. One of its favorite tactics is to possess children and then sic their zombified remains on their parents. Some go as far as to suggest it doesn't even need to feed on its hosts and acts purely for fun. It even collects trophies in the form of shipwrecks orbiting its apparent homeworld of Mugg Fallow.

Judging by the state of their homeworld, which is completely taken over with mnggal-mnggal having destroyed just about everything, it can easily take over an entire planet. And has probably done this across several worlds. It also has a very simple mechanism for spreading. Once a host is taken, their remains can easily be used as a vector for spreading the goo to others. Gnall-Gnall can also use its hosts to pilot ships, so spreading to other worlds isn't a problem either. 

An important detail to keep in mind is the mnggal-mnggal isn't a species in the traditional sense. It's actually more like a single organism. Every instance ultimately connects to the same hive mind, making it great at co-ordination to the point where zombified hosts will often act in unison. 

Luckily, it is possible, if difficult, to quarantine mnggal-mnggal. It's bad enough that the Chiss are willing to pay handsomely for anyone open to purging an infected world. Full eradication is possible in theory, but nobody's come up with a plan for how to safely approach Mugg Fallow and destroy it at the source. And even then it's hard to say if that would be enough to destroy mnggal-mnggal for good.

The Thing (The Thing)

The titular monster of John Carpenter's 1982 sci-fi horror classic The Thing was described as "the ultimate in alien terror" for a reason. Xenomorph infections are bad enough, but at least their victims can be identified with some level of reliability. Actually, dealing with the infection is another matter, but usually the presence of a facehugger makes it easy to tell who was infected. Not so much the case for The Thing. 

The Thing is an alien that not only infects and consumes its hosts, but also replaces their bodies with an identical copy. There are a few ways it can do this, but ultimately, its greatest strength is discretion. The ability to perfectly copy its host allows the Thing to easily blend into any environment and spread unnoticed. 

Once it gets going, the Thing is really hard to eradicate, because all it needs is a single cell. While an individual instance of the Thing can be defeated by a flamethrower or explosives, it remains a threat so long as any cellular activity remains. And even when one instance is dealt with, its rapid spread and built-in survival instincts make it easy for smaller pieces to break off and escape. 

But what makes the Thing especially dangerous is its intelligence. It doesn't just spread to whoever's nearby; it studies its targets and strategically decides who to infect. This intelligence can be used to exploit the inevitable paranoia of its victims and manipulate them against one another. It even has the knowledge to build its own transportation using whatever bits and pieces of machinery are available.

Left unchecked, the Thing could easily assimilate every organic lifeform on Earth in a few days at the most, and that's just the latest in what is probably a long chain of planets. Given sufficient means of travel, the Thing could spread across the galaxy and maybe even further. 

X Parasite (Metroid)

Metroid's X Parasite is a gelatinous organism that consumes its hosts and replaces them with an identical copy, similar to the Thing. In their base form, X Parasites look like large blobs, usually yellow though variations in other colors exist. In this state, they are really hard to kill, simply because it's hard to actually do any meaningful damage. But it gets worse when they actually infect a host. 

As a gelatinous lifeform, the X Parasite can easily squirm into the cracks of any armor and absorb itself into the host's body. They seize control of the host's nervous system and then begin reproducing. Each copy of the X Parasite begins taking over more and more of the victim's body, absorbing their DNA. When the process is finished, the host is replaced with a nearly identical copy. It will look like the original host, while also inheriting all their skills and memories. Just about everything short of emotions. 

Unsurprisingly, the X Parasite can spread really fast, especially if it gets the right host. The entire infection process only takes a few seconds. For the Chozo, it got so bad that they had to genetically engineer a predatory organism to keep it at bay- this is literally the entire reason Metroids exist. And even they couldn't completely wipe it out. 

Voidworms (Stellaris)

If some of the parasites above seem frightening, how about one that consumes entire planets? While many of the previous entries, like the Thing or Seed Pods, and the Genestealers, could easily take over a planet given the chance, they still have to infect on an individual level. That usually means gradually infecting hosts one at a time. World domination doesn't happen straight away, and there is at least theoretically time to identify and stop them. That all goes out the window for voidworms. 

These space leeches travel across the galaxy looking for planets occupied by biological life forms. When they find a suitable world, they bombard it with spores, quickly overwhelming and easily infecting any living organisms on the surface, whose bodies become incubators for the worm's young. Eventually, they emerge from their hosts as nymphs and leave the planet, continuing the cycle.

While they don't appear to have the intelligence of other species, their ability to mass infect an entire planet gives them a firm reproductive advantage. It is possible to develop a cure to the infection and even an immunity, but this takes time- easily enough for the infection to spread before a cure can get anywhere near completion.

Another thing that makes voidworms especially dangerous- they can travel through the vacuum of space. Keep in mind that other examples, like the Thing or even mnggal-mnggal, still need to either build their own transport or exploit their host's technology. Voidworms can travel through space unimpeded and, while not indestructible, become formidable opponents when allowed to grow to their full size.  

The Flood (Halo)

The parasitic menace known as the Flood is basically responsible for setting the entirety of Halo into motion. This parasite mainly transmits itself through airborne spores, which are then breathed in by other species, allowing their bodies to be taken over and mutated. There are several different forms that mutation can take, but they all serve the same basic purpose of spreading the infection by whatever means available. The use of spores make it easy to spread and infect multiple hosts at a time. 

That alone would be bad enough, but it gets worse. The flood considers intelligent species a primary target because it can learn from each host's mind. When a host is infected, the flood gets access to their mind. It can read their memories and access everything they know. The more intelligent hosts consumed, the more it learns. This allows the flood to become a lot more organized and strategic. Once it starts forming a gravemind, the Flood can start using advanced battle tactics and even play conflicting sides against one another for its own ends. 

This already happened once before. The flood became so widespread that the Forerunners had to turn to an extreme solution. The only way to stop it was to use the Halo Arrays, a collection of superweapons powerful enough to wipe out all life in the galaxy. While Chief is able to prevent that happening a second time, they come pretty close.

Brethren Moons (Dead Space)

The Brethren Moons are massive aliens, similar in size to the Jovian moons, who have built their entire existence around exploiting the evolutionary patterns of other species. Their method relies on what could be described as a galactic trojan horse, known as the marker. It's like a cruel inversion of the monolith from 2001: A Space Odyssey. Where the monolith acts to further the evolution of other species into an intelligent spacefaring civilization, the marker presents itself to an established spacefaring species and engineers their downfall.

The whole system relies on a common evolutionary pitfall. Each species encounters a problem when it exhausts their homeworld's resources and must travel into space to sustain itself. The Brethren moons offer the marker as an apparent solution- a free and infinite power source, and who's going to say no to that? Even better, they're designed to be easily studied and reproduced. The host species, seeing a perfect solution to their energy problems, will begin making more markers and dispersing them across every planet they've settled. All while failing to realize this is only step one of a bigger, much darker plan.

By becoming an integral part of the host species' society, the markers are able to secretly begin manipulating people around them with electromagnetic frequencies. This begins discreetly, with the marker causing people in proximity to begin experiencing hallucinations. This is used to create chaos which then leaves bodies that can be reanimated as necromorphs, corpses modified into durable killing machines that further the infection. The final stage is a convergence event where all the leftover biomass is brought together and forms into a new Moon.

The biggest advantage the moons have is their ability to mass infect on a level unseen in most parasites, without even touching their hosts. While they are certainly capable of surviving in a vacuum, they don't even need to travel. The use of markers allows them to quickly infect multiple worlds simultaneously, while also taking their host species by surprise. It's already known that they previously destroyed thousands, if not millions, of other civilizations through this process. And each cycle only produces a new moon, adding to their numbers and making them increasingly powerful. 

The only limitation the moons have appears to be a technological one. They can launch markers around the galaxy, but lack the means to actually build them, instead relying on their host species to replicate and spread them across any given civilization. But being as immensely powerful as they are, this is a pretty minor drawback.

Starro (DC Comics)

Now, admittedly, when it comes to this one, the available data can be wildly inconsistent, thanks to DC's tendency to constantly rewrite its own canon. Is he a single entity or part of a larger species? What exactly is the full range of his powers? Is he even truly a villain or more of an anti-hero? It all depends on exactly which version we're looking at across a huge range of comics from different eras, plus movie and TV versions.

That said, Starro is usually portrayed as a ruthless tyrant seeking power and control. To that end, he relies on a seemingly infinite supply of parasitic starfish that can be launched at will. These will attach themselves to a host's face and override their nervous system. The host's free will is suppressed, and their body effectively becomes a drone for Starro's bidding. 

Whether the host can survive this process depends on the version, but Starro can launch thousands of stars at once, and co-ordinate hosts across massive distances unthinkable to most parasites. At his most powerful, Starro has been able to infect and control hosts across nine galaxies. And he has even shown a capability to infect superheroes, having used his mind-control stars on several Justice League members on multiple occasions. 

The Hiss (Control)

The Hiss are an extra-dimensional entity that even managed to overwhelm an entire US government agency dedicated to protecting extra-dimensional threats. The Hiss can infect and assimilate its hosts in an instant, transforming them into tools that further its spread. There are a few different ways hosts can be used, but they all serve the same function. At best, hosts get turned into transmission vectors who constantly perform a mysterious incantation that seems to facilitate their spread. At worst, hosts can turned into weapons, their skills turned on their former allies. Or even worse, the hiss can modify their hosts on a molecular level, twisting and contorting their bodies to whatever serves their purpose.

Some of this might not sound that unusual, but how many parasites can travel across dimensions and spread by literally breaking reality? The Hiss are powerful enough to easily overwhelm and confuse an entire government wing dedicated to dealing with extra-dimensional threats, taking the majority of its personnel almost immediately. Just Containing the hiss was only made possible by intervention from another equally powerful entity, and even that took massive foresight and extensive research from their head scientist.  If not for Polaris, the Hiss would easily have overtaken the world and probably continued spreading, maybe even destroying the universe as they have in other realities.

No other entity on this list has the power to deal the amount of damage the Hiss can do when allowed to run wild. This easily makes them the deadliest parasite there is.

A Bunch of Great Warhammer 40k Game Concepts (Games Workshop, please call me)

Warhammer 40k has a long history with video games, perhaps fittingly since it originated as a tabletop game. They don't always work perfectly, but when they do, the franchise has come up with some truly impressive titles. And there has been a pretty good streak of successful games in recent years- Space Marine 2, Boltgun, Rogue Trader, even the mobile game Tacticus. But a lot of these games have barely scratched the surface of the franchise's notoriously bleak universe.

The more I deep dive into Warhammer 40k lore, the more I start to find myself noticing things that could make really interesting games, yet remain untouched. I start to have really cool ideas for video games that sound really interesting, if only I actually understood game production and knew how to get the rights. So I'd like to present a collection of ideas I've had for 40k games that I think could make something really interesting. 

Games Workshop, if you're reading this, please take these ideas. Take all of them. Take them and put your best people on them. You don't even have to pay me for suggesting these; giving me credit is enough. Just make them happen!

Note: I'm deliberately avoiding any ideas centered around Space Marines. While there is certainly lots of untapped potential around them, they have been done so much already, and I'd like to propose something different.

Adeptus Custodes Hack n' Slash

It's not hard to see what makes the Space Marines so interesting to Warhammer 40k fans. They're genetically augmented super soldiers built to take on the worst of an unforgiving galaxy. But if we're going to go that route, why not go even bigger? After all there is one faction that is even bigger and tougher than Space Marines- the Adeptus Custodes. These golden-armored warriors are usually around 9 ft tall, easily dwarfing most Space Marines. And when they appear, it's a sign that things are really bad.

If Space Marine 2's main selling point was playing as a genetically augmented super-soldier, playing as a custode would take that to a whole new level. Players would get to be the one thing that's bigger and tougher than a space marine (at least within the ranks of the Imperium). This would mean being able to blast through monsters that could easily overwhelm someone like Titus. 

Another fun detail about the Custodes is they are the Emperor's personal guard, so playing as one would be an opportunity to bring one of 40k's key locations outside the realm of concept art and into an immersive 3D environment. The Imperial Palace is the seat of power for all of humanity. A Custode game could actually explore this place and some of its mysteries, maybe even feature an appearance by the Emperor himself (or what's left of him, anyway). 

That said, there is an aspect in which a Custode game could do more than just feel like playing as super space marines, and that would be focusing on their differing combat style. Space Marine 2 allowed a pretty solid range of melee weapons, but it was ultimately a shooter. A custode game would be the opposite. 

While they are definitely capable of using firearms and explosives, the Adeptus Custodes primarily use melee combat. So a game centered around them would logically be more of a hack-and-slash than a shooter. This would create a much more distinct experience from the Space Marine games. 

While Custodes could theoretically work at any point in the timeline, a game centered on them would be a great opportunity to explore the Horus Heresy, a Civil War which came closer than anyone before or since to destroying the Imperium. Playing as a Custode experiencing one of the biggest genre-defining moments of the franchise would be a great selling point.

Adeptus Mechanicus RPG

The Adeptus Mechanicus is one of the more peculiar factions in the Imperium. They are a techno-cult from Mars that has a... peculiar relationship to technology. They'll build marvels of engineering but also feel the need to bless it with incense and elaborate rituals. This goes beyond mere superstition, with "machine spirits" actually being a thing in this world, but it definitely looks strange to any modern engineer. The only thing is that they can't actually innovate, at least not openly. Instead, they have to scavenge and rediscover old pieces of lost technology. 

And here's where things get messy, since one of Adeptus Mechanicus' core beliefs is the inherent weakness of the human body and the need to ascend through technical augmentation. On the more moderate side, this basically means digging circuit boards out of the trash and shoving them into your body to see what happens, and becoming a cyborg. The more fanatical ones do that so much they don't have a body left, becoming more like a brain encased in a machine. 

Now the Mechanicus have appeared in games before. They did have a notable role in Space Marine 2 and, unsurprisingly, they starred in Mechanicus 1 and 2. But there hasn't really been a game that's taken full advantage of their strange beliefs. An RPG could have some fun with this, casting the player as a new initiate into the cult and putting an emphasis on scavenging parts. An RPG could allow the player to experiment with different types of upgrades, essentially customizing their augmentations for different playstyles. We've seen this kind of thing work before in games like Cyberpunk 2077, though a mechanicus RPG would probably be a lot more extreme in this regard.

Another fun detail- there are some interesting philosophical questions that could be explored with this setup. Questions about what it means to be human, and how far the player is willing to go to achieve their full potential. This could even be a mechanic in the game, where the player is given the choice to dive deeper into mechanical augmentations or try to retain some of their humanity, with pros and cons both ways. 

Cadian Tactical Shooter

On the flipside, it might be refreshing to do a game where you play as a regular human. Let's face it, sci-fi games centered around playing a genetically augmented supersoldier are becoming a tired cliche. From the Space Marines to Halo's Master Chief, it's been done a lot. What about changing things up and instead playing a game where you get the experience of being one of those supporting NPCs who lack all the fancy modifications. 

In the context of Warhammer 40k, even the normal humans have to be tough to survive its unforgiving universe. And few exemplify that resilience quite like the Cadians. As many like to say, their homeworld broke before they did. The Cadians are easily among the most versatile and experienced factions when it comes to warfare, yet they often get reduced to the background as support for Space Marines. Putting them front and center would offer a unique experience.

For one thing, playing as a Cadian would allow players to see the 40k universe in a new light, since we could see how it looks to a regular human without all the fancy gene seed upgrades. This would make a lot of encounters more daunting, and more challenging. But that opens up a new angle for a Cadian game- greater focus on tactics. After all, Warhammer 40k is supposed to be a tactical game, is it not?

A logical way to do a Cadian shooter would be to put an emphasis on caution and strategy, possibly following in a similar vein to the Ghost Recon series. The player is cast as the leader of a Cadian squad tasked with daunting objectives. You can't just go charging in and opening fire, you'd actually have to evaluate the situation and determine the best strategy. It would be harder, but that might make it a lot more satisfying when the Cadians actually manage to take down a Carnifex.

The great thing here is Cadians are trained to be versatile, which leaves room for the player to experiment with different playstyles. They could experiment with more aggressive techniques or go for a quiet stealth-based approach. It could even mean experimenting with different weapons, vehicles, and squadmates to find the right combination. 

Death Korps of Krieg

The Death Korps of Krieg is an intriguing faction with a messed-up history. Long ago, there was a rebellion that led to a civil war. And now the descendants of those who fought at Krieg are forced to perpetually "atone" for their actions. And of course, atonement can only be achieved by fighting and dying for the Emperor. 

This often gets exaggerated into the Krieg recklessly charging into danger in the hopes of getting killed, which isn't entirely correct. More accurately, they're trained to work in hostile environments and produce maximum results with minimal resources, and conditioned to be willing to put their lives on the line if the situation calls for it. 

This is actually a pretty good setup for a game. A campaign centered around Krieg could emphasize three major things- squad-based gameplay, brutal fighting conditions and limited resources. This would make combat a lot harder, since the player would have to find unconventional ways of dealing with problems and occasionally improvise or use the environment to their advantage. 

Another thing about Krieg is that they have a very distinct aesthetic that would help the game stand out. One of the big things that allowed this faction to resonate with players is their WWI-inspired design, which extends to their weapons and tactics. A skilled design team could really take advantage of this to create some amazing visuals.

There is also a practical side to doing a game about the Death Korps of Krieg. They almost always wear gas masks, so focusing on them would mean significantly less work on facial animations, something that's often a daunting prospect even under the best working conditions.

Orders Hospitaller Simulator

Orders Hospitaller is a unique faction within the Adeptus Sororitas, being one of the few orders that aren't combat-focused. Of course they can still be ruthless towards enemies of the Imperium, and being capable of self-defense is essential in 40k's unforgiving galaxy, it's just not their primary objective. Instead, they serve as the closest thing 40k has to first responders, providing valuable medical care and aid relief. And this could actually make an interesting spin on the franchise.

It comes as no surprise that war plays a big part in Warhammer 40k, but there is a lot more to fighting a war than just shooting enemies. Like, for example, being able to provide medical aid to soldiers. And this would be where a game centered around Orders Hospitaller could do something interesting. It would still fit thematically- there is still war happening, the player is an active part of the war effort, and they are helping people who have been affected by war.

A game centered around Orders Hospitaller would shift gears by moving away from front-line combat to acting more like search and rescue or field medics. Gameplay would probably be focused more on navigating war zones and treating wounded soldiers or rescuing civilians from disaster (which, again, was probably caused by all that war). 

 Of course, it wouldn't need to forego combat entirely. No doubt the protagonist would occasionally need to defend herself against any number of threats. There could even be moments where she has to take up arms to protect her sisters or patients, or occasionally deal with a heretic in the ranks. The point is, it would be a game about war where shooting wouldn't be the central focus. While perhaps an unconventional approach, it actually could make for a truly unique game within the franchise.

Sisters of Battle Shooter

Space Marine 2 was a huge success, and a masterpiece in a lot of aspects. But it did have the downside of being very male-dominated in a franchise that already has a reputation for hypermasculinity. There weren't a lot of female characters in the campaign and the ones it did have didn't get a lot of screentime. This does make a degree of sense, seeing as its primary focus is on the Astartes, a faction that is explicitly all-male in the lore. While it might have been nice to see more of Major Sarkanna, it was ultimately Titus' story. 

However, what if there was another faction we could do that might be a little less... masculine? Something similar enough to the Space Marines to adopt a lot of the same gameplay but distinct enough to still be its own experience? Like maybe a female counterpart to the Space Marines... oh wait, there is! The Sisters of Battle are the closest thing Warhammer 40k has to female space marines, and they're sometimes framed as a counterpart. 

A game centered around the Sisters of Battle could likely work with a similar structure to Space Marine 2. You could have your central protagonist accompanied by two of her sisters, with an emphasis on the dynamic between them and optional co-op. They use a lot of the same weapons as well. You would still have your bolters, flamers, grenades, chainswords, so gameplay wouldn't be too far off. 

But there is one thing about playing as the Sisters of Battle that would make it distinct from playing as a Space Marine- you're not playing as a super soldier. While well-trained and ruthlessly effective, the Sisters of Battle lack the genetic upgrades of their male counterparts. This game could take advantage of a change in scale, adopting similar basic mechanics while recognizing the Sisters are around half the size of titus and therefore need to adopt different tactics when taking on the same enemies. And managing to take down something like a carnifex that is usually able to go toe to toe with a Space Marine would definitely feel pretty satisfying.

Sisters of Silence Stealth Game

The Sisters of Silence are a curious faction who certainly live up to their name. The name has a double meaning. For one thing, they are literally silent, as in they never speak. Becoming one requires an extreme vow of silence. They also act as assassins who rely on stealth and cunning to reach their targets, rather than a direct assault. And this is where the potential starts to manifest. There hasn't really been a Warhammer 40k stealth game, and the Sisters of Silence are practically tailor-made for such a project.

The obvious structure would be to do something in the vein of either Hitman or Dishonored, where the player is assigned an objective and placed into an open-ish world that leaves them different options on how to accomplish it. The Sisters do come with some crazy powers that could also be incorporated into gameplay, most notably they can disrupt the powers of psykers just by being in proximity. How is this not already a game?

The other fun thing with the Sisters of Silence is an opportunity to explore one of the key defining moments of 40k's history. While this could work at any point in the timeline, they had an especially prominent role during the Horus Heresy, a brutal Civil War that broke the galaxy. Playing as a sister of Battle who does sabotage and assassinations against Horus's forces would be a great framework for exploring such a defining moment in the lore.

Tallarn Guerrilla Experience

The Tallarn Desert Raiders are an underrated piece of the Imperium that doesn't get much attention, which is unfortunate because they are quite unique. Inspired by the Fremen of Dune, Tallarn is built around guerrilla warfare, which could actually be a pretty good setup for gameplay. Aside from bringing a lesser-known faction into the spotlight, a game centered around Tallarn would still fit with the themes of Warhammer 40k but offer a very different style of combat to other factions. 

With their specialty being in Guerrilla Warfare, a Tallarn-focused game would logically be put more emphasis on stealth and strategy. Players would need to carefully evaluate each objective and figure out the best way to get maximum results quickly and efficiently. And, depending on the circumstances, there might be various ways of doing that. Perhaps a bit of sabotage to weaken their defenses is in order, or a carefully timed explosion, or maybe you just need to know where they're going to be and set a trap.

Ghost Recon Wildlands might be a good point of reference for how a Tallarn game could work. A squad-based game where the player has to strategically make use of the available resources is a good approach. Maybe they could have a few different specialists to choose from for different strategies. Alternatively, something more in the vein of Sniper Elite, where the player is thrown into the meatgrinder and left to experiment with different strategies could also work.

Tomb Raider-Style Action Adventure Game

The universe of Warhammer 40k has a pretty complicated history, one full of mystery and dread. Just among humans, there's the whole Dark Age of Technology before the rise of the Imperium (which itself has existed for a few thousand years in some form). But the full timeline goes back well before humanity existed, with the earliest recorded event being a galaxy-breaking conflict known as the War in Heaven. And there might be some things even older than that, like the enigmatic Blackstone Fortress.

The point is the galaxy has a crazy past and uncovering old ruins and relics is already a common theme in Warhammer 40k, so why not use that as a framework for exploring some of that history in more detail? An action-adventure puzzle game in the style of Tomb Raider or Uncharted could be a fun way to do that, while also offering something a bit different. 

The premise could involve an archaeologist exploring old ruins while searching for a powerful artifact. It could be a human researcher seeking technology they think would aid the Imperium, or maybe a mechanicus tech-priest looking for knowledge. Either way, they would be searching through ancient structures and uncovering pieces of history, and, as happens a lot in 40k, a much greater danger. 

As for what they're investigating... well, take your pick. The Horus Heresy is already a thousand years ago at the current point in the timeline, and that's on the more recent end of things. There's the Dark Age of Technology, Necron Tombworlds, the Blackstone Fortress (itself an enigmatic mystery that could work great with Tomb Raider-esque puzzles), and anything leftover from the Aeldari empire.

But if you want to go really far back, why not use this as a way to explore the War in Heaven? It basically set everything in motion, but it's never really been shown. This approach could be an interesting way to tell a story around that conflict. Some of that fancy ancient technology could even be used to create flashbacks and let players experience actual moments of the infamous conflict.

And it might sound weird to do a puzzle game in Warhammer but that could still work. There could still be conflict, perhaps our protagonist is dealing with an opposing faction or one of the many xenos who are also competing for the same artifact? But thematically, it would still make sense on the grounds that it's exploring how war shaped the galaxy.

Tyranid Hive Simulator

Of all the alien species introduced in Warhammer 40k, tyranids are easily some of the most terrifying creatures. The easiest way to describe them would be something like the Xenomorphs of Alien, except somehow worse. They are a predatory hive mind whose only objective is to consume everything in their path. But the real danger is their intelligence. While their goals may be simple, tyranids are capable of elaborate battle tactics and espionage, and they have access to some advanced technology. And this is where a game could find some interesting opportunities. 

The thing to really take advantage of is the hive mind. In a way, the player is controlling the whole swarm. A fun way to do this would be to cast them as a hive tyrant, a sort of controller organism that acts like a commanding officer. Except instead of just giving orders, playing a hive tyrant would make it possible to directly control, and swap between, different types of tyranids. This would allow an interesting approach that could mix a shooter and real-time strategy format and put a lot more emphasis on their tactics.

There also hasn't been much that actually shows their perspective. Tyranids are usually portrayed as antagonists, so it would be an interesting shift to see what it's actually like to be them, especially if it had made a compelling story that actually makes the hive somewhat relatable, or at least understandable. Or at least allows players to have fun controlling supersized monsters and terrorizing the galaxy.

Imperium Team-Up

A common thread through all these suggestions is that a lot of them are, ultimately, part of the same Imperium. We have all these different groups we're  used to thinking of as separate factions- the Space Marines, Custodes, Sisters of Battle, Cadians, Death Korps, etc. But at the end of the day are they not all serving the same emperor? Sure, they may still have drastically different values and ideas of how to serve the Emperor, but that could still be a good source of conflict for a story. 

So why not bring a bunch of them together? Not just as different types of armies, but as a character-driven story around people brought together by unexpected circumstances who have to settle their differences for a common goal. Here's the setup: the Imperium sends a bunch of its forces on a critical mission, but things go bad (as happens a lot in 40k). Almost everyone is killed, save a few survivors who are reluctantly brought together and realize it is there duty to finish the job.

So here's what I'm thinking- a shooter using a similar structure to Borderlands where there are multiple playable characters and options for both single-player and co-op. Each playable character would come from a different part of the Imperium and therefore be tailored to a different playstyle.

So if, for instance, the playable roster included members of the Space Marines, Sisters of Battle, Cadians, Death Korps, and Tallarn Desert Raiders, each one could have different strengths and weaknesses. A Space Marine could be good for confronting larger enemies, while a Sister could be more agile and better at targeting weak points. Meanwhile, a Tallarn desert raider could focus on support through stealth and ambush tactics, and a Cadian could be portrayed as versatile.

And as far as the story goes, there's a lot of potential conflict just from the characters settling their differences. There is, canonically, already a rift between Sisters of Battle and Space Marines they would need to overcome. The Space Marine's interactions with the others could vary widely depending on which chapter he belongs to. An Ultramarine or Salamander will be a lot more amenable to working with normal humans than a Black Templar or Blood Angel.