"Iä! Iä! Tyrannicus fhtagn!"
- – One of many unholy Genestealer Cult prayers
"Praise be! The star children deliver us!"
- – White Dwarf October 2016
"Mankind has always looked to the stars for salvation...and finally, THE STARS HAVE ANSWERED!"
- – Unnamed Magus
Genestealers are alien creatures in the Warhammer 40,000 universe. They have undergone many fluff revisions since their inception, but they have always been lethal in melee (so deadly that Space Marine power armor is like tissue paper to them), preternaturally fast, pseudo-insectoid in form, and reliant on other species for reproduction.
Basically what would happen if the Xenomorphs decided to run a Lovecraftian doomsday cult... IN SPAAAAACE!
In Warhammer 40,000: Rogue Trader (the first edition of Warhammer 40,000), Genestealers were simply one of many creatures encountered in space, spreading from the moon of Ymgarl. They could be extremely dangerous in close quarters, as each of their six strong limbs ended in sharp claws, and they had a gruesome lifecycle reminiscent of the Xenomorphs from the Alien franchise, but otherwise had little to distinguish themselves. Most notably, they were not connected with the Tyranids at all. Indeed, the core rulebook noted that Genestealers with human ancestry could be intelligent, and even friendly! Their natural form had an almost leech-like head, and they were specifically described as "vampirish." They also ignored armour saves until 4th ed. That was annoying.
When the Space Hulk board game was released a few years after 40k: Rogue Trader, Genestealers received a significant bump in their threat level. The game made it clear that the Genestealers were a much more virulent and widespread menace than their initial description, and their head was changed to have a much toothier mouth. In keeping with GW's ripping off the Xenomorphs, Genestealers would use stealth to approach the Marine player's Terminators as "blips" of some unknown number of Genestealers, and then come out of hiding once they had a good ambush prepared. They achieved a super-human level of coordination via a hive-mind, represented by the Genestealer player having unlimited time to move his pieces.
In later editions of Warhammer 40,000, the Genestealers were revealed to be (or retconned into, depending on your point of view) vanguard organisms for the Tyranid Hive Fleets. They are capable of thinking for themselves and operating without the Hive Mind's leadership, a rare trait among Tyranids, although they are not synapse creatures. Space Hulk-era Genestealers were re-designated "Purestrain Genestealers," created by the Hive Fleets themselves.
Genestealers are sent to infest Space Hulks and spread among the stars. Their long tongues contain a barbed ovipositor; when they encounter sentient humanoids, they use this ovipositor to inject a "seed" into the host's body, combining the host species' genome with the greater Tyranid genome (hence the name Gene-stealer.) This is a method called the "Genestealer's Kiss," which is either a face-biting parody of a kiss or an injection under the ribcage. More recently, the Genestealer Cult of the Twisted Helix has discovered how to extract this "seed" for use in contaminating food and medicine, allowing the Genestealer curse to spread to locations far removed from any actual Genestealer.
Regardless of the vector, the seed greatly alters the host's body on both physical and psychological levels over the course of a few hours, causing them to forget all about the infection and become subservient to the Purestrain's brood. The host is also driven to have children, even if they didn't want any before becoming infected; their partner does not have to be likewise infected, but it doesn't hurt. After a pregnant host (or whoever they impregnated) gives birth, their viciously malformed offspring creates a hive mind connection between itself, its parents, and the Genestealers (though this raises the question of how anyone else in the room during the birth, such as midwives, would react.) This connection and the mutations brought on by the Genestealer's seed proceed to subliminally twist the minds of both parents so that they unconditionally love their child and revere the Genestealers either as gods or as creatures sent by gods (the exact interpretation varies, you understand), usually fleeing into darkened tunnels, catacombs or sewers to avoid discovery. It is not even uncommon for Genestealer Cults to present, at least outwardly, as Emperor-worshipers, although inevitably they have some funky iconography like the whole four arms thing. Some of them, at least the rank and file, may even believe they're worshiping the Emperor instead of spacebugs. And, when not doing dastardly Genestealer things, they may really be about the overall work of the Imperium on a day-to-day basis, which is of course part and parcel with their infiltrationist aspects.
Eventually these infected individuals and their growing brood of offspring will come together with kindred spirits (that is, other Genestealer hybrids and their infected parents) to form a community/family dedicated to the Genestealers in both body and soul. As the family grows, they will continue to isolate themselves, eventually forming a Genestealer Cult. What follows is an ongoing process of hybrids breeding with captured and infected humans, with their human parents kept around both as breeding stock and for nursing and child-rearing. The second generation hybrids aren't as ugly as their parents, appearing more "normal"-looking but still obviously alien. However, that becomes a moot point once the third and fourth generation roll around, the fourth actually being virtually indistinguishable from actual full-members of their parent species. The uglier members and Purestrain Genestealers typically lurk in the shadows far away from civilisation, while the intelligent and "human" members infiltrate and spy in almost every sector of society. Their psychic network lets them communicate with one another, even though some of the less-human hybrids (and the Purestrains) are incapable of speech, and when the brood mind becomes powerful enough, it will act as a beacon to Tyranid Hive Fleets. These fleets travel at various different speeds according to various different authors although generally always slower than Imperial warp drives, which means it is impractical for them to travel blindly, so they home in on the signal created by a large Genestealer Cult. When their arrival is imminent, they will directly contact the Cult, which will then engage in a full rebellion, sabotaging their planet's defenses in preparation for the fleet. The fleet will consume the Cult along with everything else on the planet; how the Genestealers and their hybrid servants feel about this varies from edition to edition, and sometimes even planet to planet. Sometimes cultists will desperately fight back, while other times they will accept their destiny with utter joy.
The exact manner in which a Cult reacts to the arrival of the Tyranids varies. According to the backstory for Hive Fleet Kraken, a planet called Larnarno was infiltrated by a Cult calling themselves the Celebrants of Nihilism. When the Tyranids showed up to consume the planet, 75% of the population calmly marched towards and aboard their bio-ships, where they gladly accepted being brought to the digestion pits. However, a Cult generally has no idea what is about to happen to them, fully believing that when the Tyranids show up they'll all become one big happy space family. And for a bit, the Hive Mind actually leads them on and doesn't target the cult. For a brief moment, the cultists get to live their dream of fighting alongside their star gods, but as soon as the planet's defenses are basically done for though, everyone is on the menu. When the cult realizes this, they panic, their faith completely destroyed. Even worse, the Hive Mind overrides the independence of the Patriarch and Purestrains, and the cultists that try to flee are consumed by the very father they worshiped and the very children they raised.
Some may ask, why would GW do this? Does GW hate the idea that anyone who is not of an Imperial faction might be allowed to have a happy ending? If you have to ask these questions, you probably don't understand the situation. The Genestealer Cults are literally nothing more than convenient tools that the Tyranids use to prepare worlds for consumption. The very genetic fabric of each Cult member, right down to the most lowly cultist, has been hijacked to achieve this end. If they fail, they will be destroyed by the civilizations they are attempting to infiltrate. If they succeed, they will be assimilated by the Hive Mind. Therefore for the Genestealer Cults, one way or the other, there can be no happy ending.
Way back in the days of Rogue Trader, Genestealer Cults also used to be able to be devoted to Chaos. That disappeared from the tabletop along with the old incarnation of the army, but in the fluff, this can apparently still happen, in certain very rare circumstances. One account describes a truly hilarious case of turnabout when a Cult's ship is sucked into a Warp storm and dumped on the edges of Nurgle's Garden, where the Genestealers and their minions get taught a thing or two about real corruption and parasitism by the Chaos God who wrote the book on it. Nurgle does eventually let the Cult go, because he's a nice guy, after he's thoroughly had his way with them of course, and they come back into realspace as basically a Plaguestealer Cult. What might've happened if the Cult had fallen into Slaanesh's domain, officially, we'll never know, because family-friendly GW doesn't have the balls to go there. Unofficially, we can make a pretty good guess. As a contrariwise example, there's a genestealer cult in a Inquisitor Czevak story who, marooned on a daemon world in the Eye after their space hulk was unfortunate enough to crash there, nonetheless hate and resist chaos and maintain a lot of Imperial iconography and so on (although they do consort with xenos and mutants, which, while heretical, is probably still well above par for the course given their living situation).
Interestingly, it appears that the rare Genestealer Cults that fall to Chaos worship may become decidedly unappetizing to the Tyranids, to the point where a hive fleet will ignore the cult's call and merrily skip past them to go chow down on some forest-moon instead of the world the cult had been desperately calling them towards. This ends up pushing a cult even further into Chaos worship, unaware that they are not in fact following the Hivemind but instead possibly a demented purple whore or a bloated pus-covered maggot man.
The first Genestealer to infect a host (or hosts) grows larger and more intelligent with each mind added to the cult/family/whatever, eventually far surpassing its kin in strength, size and cunning, even becoming a powerful psyker. This Genestealer is known as the Patriarch, and acts as both the cult's leader and the object of its devotion. The Patriarch's psychic connection to the Hive Mind allows it to maintain iron control over his brood as well as other lesser Tyranid organisms.
After the Purestrain infects a host, a Genestealer Cult will end up with members with various levels of Genestealer ancestry. In general, Genestealer Hybrids will breed with their non-Genestealer parent species, and their descendants will resemble that species, until the fourth hybrid generation, whose children will be Purestrain Genestealers capable of "founding" new Genestealer Cults.
The Rogue Trader-era Genestealers also get a special mention, in the form of "Ymgarl Genestealers," a strain that supposedly comes from a population of Genestealers that got isolated on the moons of Ymgarl. They have tentacled, lamprey-like mouths and the ability to partially transform themselves depending on the circumstances, at the cost of being genetically unstable and only being able to feed on blood. In fact, they're so unstable that the Hive Mind deliberately abandoned them for fear that they would contaminate the Tyranid gene pool. Because of this, they compulsively seek to reach planets that have recently been visited by the Tyranids in a futile attempt to be reabsorbed into the Hive Fleets. If they weren't horrible monstrosities, their predicament would almost be sad. However, at some point between 5th and 6th Edition, the Ymgarl Genestealers lucked out and got absorbed into a Hive Fleet by some very desperate/caring Norn Queen. The result of this is that 6E Tyranids now have a Bio-Artefact (If you feel unclean about the name, that's natural) called the Ymgarl Factor that gives any Tyranid unit the same unstable properties rule, but now lack the assaulting from Outflanking.
As a final note, Warhammer 40,000 fiction is very human-centric, and so most Genestealer infestations depicted have occurred among human populations, but the Ciaphas Cain novels have suggested that Tau and Orks are also susceptible to Genestealer infection. The novel Death of Integrity also confirms that Genestealers will attack and assimilate other xenos, though this plot point lasts about one paragraph. What, exactly, a Tau Genestealer Hybrid would look like is the subject of much speculation. There is old canon art of Ork Genestealer hybrids, but given the vast amount of retcons since then its canon status is questionable..
It should be noted that Orks reproduce asexually by way of fungal spores they secrete both at a perpetual slow trickle from their skin while alive and en masse while a decaying corpse (and also that "ork" is but one of many many different orkoid phenotypes that a given spore might develop into depending on environmental conditions, so one has to wonder how the genestealer mutations might interact with a spore that grew into, say, a mushroom), so it's pretty hard to envision an Orkified Genestealer Cult actually working, or lasting beyond the first generation or two, since they have no ways of mixing genes to produce purestrains in time. Even if they could, the Orks have an inherent ability to determine if one of their own isn't proppa Orky and will inevitably krump the gits. Some old, old fluff portrayed Purestrain genestealers descended from Ork hybrids as having denser musculature (and most likely greater physical strength). In another of these dusty accounts, an incident on the fringe of the Octarius system had a massive, purple, six-limbed Gargant attacking a Guard regiment. When it was melta'd open, purestrain genestealers emerged to eat the Guardsmen. You are encouraged to take these older bits of fluff with a generous pinch of salt. In any event, ancient and likely non-canonical fluff aside, the unique quirks of Ork biology boil down to the fact any attempt to infest an Ork warband would almost certainly fail and not be worth the effort in the first place.
Tau, similarly, have certain complications which may prevent Genestealer infestation. Firstly, their Kroot allies can tell if a person is infected or not simply by tasting some of their blood, and secondly, tau-tech is certainly advanced enough to scan for this threat once they know about it. Furthermore, Tau explicitly practice caste-restricted bureaucratically arranged breeding processes; Tau don't have kids until their higher ups decide they should, and they don't even get to pick their own partners. Two tau get paired up by the decision of a committee, spend a couple of days off work and having sex, then separate and go their separate ways. This means the genestealer taint is seriously difficult to spread amongst the race. The caste-based structure of tau society also poses problems to any potential Tauified Genestealers, and that's without presuming the caste-system is now so inherent that different castes can't even physically breed with each other anymore. All that being said, according to the new Genestealer Cults Codex, not only can Tau-Genestealer hybrids happen, but one time they were deliberately created by a team of Earth Caste researchers who wanted to see what would happen. The infection got out of control and actually ended in violence. It's even implied that they may have produced a hybrid Ethereal, which would be a disaster for the Tau given how obedience to the Ethereal caste is basically hard coded into their DNA.
While it is possible for Genestealers to infect the Eldar, it's extremely hard for them to get anywhere with any of the four varieties due to their highly advanced medical technology (all four, yes even the Exodites), universal emphatic abilities, very regimented lifestyles (Craftworld), slow breeding rate and the fact that irregular psychic activity amongst them gets located and dealt with very quickly (all four, heck mind bulletry is pretty much the only thing the Dark Eldar ban). Even when they are infected, the taint spreads much more slowly due to the Eldar's prolonged gestation period, which makes infecting them more trouble than it is worth in most situations.
So far the only examples among Eldar are those who knowingly and openly embraced their pants-thieving overlords. In the novel "Ghost Warrior", we have Genestealer Eldar on a lost craftworld named Zaisuthra. Needless to say, the other Ynnari who contacted them were pissed. This was done in their misguided attempt to avoid getting consumed by Slaanesh because this Craftworld had left before the Infinity Circuit was invented and thus didn't have one; their logic was that if they gave themselves over to the Broodmind, their souls would be assimilated before Slaanesh could consume them. There is also something of a Genestealer cult growing among the Dark Eldar in Commorragh. After a raid with lots of captives taken to Commorragh, the Haemonculi found out some of them were Genestealer cultists. The hybrids were singled out and experimented on so their mutations would emerge. When some bored Dark Eldar among the social elite found out about this, they paid the Haemonculi to graft these Genestealer body parts on them for kicks (given how Genestealer reproduction involve a genetic re-write of infected hosts using the Stealer's own DNA, you can see what a stupid idea this is). It looks like the Hive Mind might find a way into Commorragh given how these Dark Eldar, who collectively call themselves the Vorgani, started forming tight-knit groups and share a singular obsession over a Tyranid-infested planet trapped in the Webway.
As for the Necrons, they're goddamn undead robots. If you even consider this possibility you're fucking retarded.
Finally, and perhaps most troubling, is the mention that Tyranids have been targeting Hrud warrens. Hrud lack the defenses of the above races and are naturally predisposed towards infesting a planet and migrating en mass to a new one. The genestealer hybrids, likewise, gain enhanced stealth and reaction speed (such as those 5++ saves the 'stealers now enjoy.)
Dawn of War 2
Genestealers are pretty lore accurate in Dawn of War 2. They eat whatever they face. They act as the ultimate melee troops for Tyranids in tier 2. Let's run some numbers: A regular Tactical Marine starts with 60 melee skill, a standard melee squad starts with 70. These motherfuckers are starting with 80 by default! And this even gets worse if they have any synapse support. If they are under Improved Synapse of the Hive Tyrant, their health increases by 25%. If they ever get into the melee synapse of Warrior Brood, their health increases by 75%, and melee skill by 10, and you can combine these 2 synapses together! Yeah, they can basically murderfuck anything in melee combat. And this gets even worse if they use their Adrenal Rush ability. Gains increased speed and damage, gains small amount of health in each hit they make, and decreases incoming ranged damage.
Everything aside, the most simple solution to deal with them is to get a walker out. Genestealers can't purchase their anti vehicle klaws until tier 3. That makes them balanced right? I hope so. But when they get them, they can solo a Dreadnought alone! Thankfully, they have light infantry armour, which means that they can be killed easily with most ranged weaponry and struggle against melee fighters that are able to survive long enough to get a few hits in themselves. AoE melee attacks are your friend against them.
On the Tabletop
TyranidsLand Raider due to their massive number of good strength attacks, rending, and high weapon skill that means they hit most things on 3s (seriously, these guys have higher weapons skill than a lot things made out to close-combat experts), not to mention that they are fast little fuckers. A Zerg rush has nothing on these guys. In the fluff, their claws are so damned sharp that they might as well be power weapons (back in 2nd Edition and pre-codex 3rd Edition they WERE power weapons), slicing through armor and walls like a chainsaw going through a rice paper wall. Their hypnotic eye powers show up irregularly in the fluff, showing up in the Dark Disciples Novel but not Ciaphas Cain; Hero of the Imperium for example. The fact they are faster than Eldar lets them laugh at any unit they charge/charging them, and they are usually deployed a-la Kroot Carnivores (parking them behind a windowless wall 12-18" from the enemy). Unlike Kroots though, they are not there to slow the enemy, they provide a hazardous area for the first turns and then, when the main force has reached their position, they jump out to help OMNOMMing.
In some editions Genestealer broods could upgrade one of the members to a Broodlord Broodlord, which formerly had a major role due to the biomorphs it could use and psyker powers, but being reduced to a sergeant basically made it just add an extra punch, though it still had a better statline than any non Special-Character Space Marine HQs (except the CSM's daemon prince). Sixth edition turned Broodlords into rape machine in challenges, as they able to bring down almost any independent character in the game without eternal warrior, while costing much, MUCH less.
In 8th Edition, Genestealers are still great, but other armies have some melee options that are pretty on-par with them. A horde of Ork Boyz, for example, can pump out 30 more melee attacks in a turn, and cost 110 points less. Granted, they don't have the 5++ invuln, but they still will wreck a horde of Genestealers' shit if they get the first attack. On the other hand, the Genestealers are also very speedy and can rocket to the other side of the board with the right Stratagems. They can also charge after advancing (or lose that ability in exchange for a better armor save that can go up to MEQ-tier if in cover or under the benefits of Jormungandr's Hive Fleet Attribute) and can take Acid Maws for a few extra MEQ-killing attacks. Oh, and they also get a Deep Strike similar to a Terminator's teleport homers. They're especially potent with Hive Fleet Kraken since they'll have better advances as well as the ability to charge after falling back, which not only saves them from running the risk of being trapped in melee with something too tough for them to deal with but also improves their already impressive maneuverability and ensures that they'll always get the first swing in.
Cults get a super-genestealer called a Patriarch, the progenitor of the cult, as the main HQ choice. There's also the Magus, acting as the cult's psyker, and the Primus, their version of a general/champion. You also get regular genestealers, though they're actually much better than Tyranid genestealers as they're hardier and stealthier. The troops themselves are roughly similar to Chaos Cultists, Neophyte hybrids being armed with guns and improvised equipment, with Acolyte hybrids fighting better in close combat. The Cult can also steal Guard vehicles, namely the Leman Russ Battle Tank, Chimera, and Sentinel, as well as their version of a Technical called the Goliath Truck. You can also just straight up steal Guardsmen and field them as Neophyte Hybrids. All cult members specialize in ambushing opponents, and get major bonuses from their leaders, but much like the Tau they fall into disarray should their HQ be killed. However, this is alleviated by the fact that HQ automatically pass "Look Out Sir!", meaning so long as you have enough meatshields you can keep them alive (hilariously enough, this can even be used in challenges for some serious trolling). And since cult leaders can summon in large numbers of reinforcements, you could be in for some real foot-slogging.
Genestealer Cults are able to ally with Tyranids (though, as mentioned before, they're not Battle Brothers as the Tyranids view their cultists as dessert rather than the main course). Curiously, they can also ally with Guardsmen, as a way to represent the fact that they will infiltrate Guardsman/PDF forces prior to the invasion, so if you feel so inclined, you can augment your forces with even more Guard vehicles.
- Fly Lords of Terra Genestealer Apoc codex 
- So not only do genestealer cults have a codex now, but this man, Chris Showers, actually made an army of them! You'll have to Google Search them yourself, though, since the GamesWorkshop website *cough*  *cough* is slowly deleting anything that isn't the webstore.
The Plague Returns
Guess who's back, bitches! http://www.spikeybits.com/2016/02/breaking-new-genestealer-cult-rules-pictures.html
Deathwatch: Overkill started us out with rules and models for cultists, in this case various hybrids wielding mining equipment and a general leadership structure of a Genestealer patriarch, with hybrid preachers and enforcers. Then GW announced that they were doing a full codex, and if early impressions and rumors are anything to go by, it's gonna be fun. Genestealer Cults are gonna be getting their own psychic discipline with 7 powers, 8 formations, wargear options, their own vehicle (a limo no less!), and, it's looking like they'll have access to Imperial Guard vehicles too. Genestealers and hybrids catching a ride in Chimeras, or advancing alongside Leman Russes? Oh hell yes.
- Warhammer 40,000/Tactics/Genestealer Cults(8E) - a general overview of the new army
- Genestealer Cult Creation Tables - why not generate your own Cult?
- Jeanstealer - One of /tg/'s creations. A rather confused Tyranid genestealer that is only driven to steal denim trousers.
- Hivestrain Azure- A fapfic about a particularly unusual strain of genestealer, likely part of a cult tainted by Slaanesh.
- Genestealer Cult Rules 40k Rules for the new Genestealer Cult from Deathwatch: Overkill
|Leader Organisms:||Broodlord - Hive Tyrant - Swarmlord - Tyranid Warrior|
|Small Creatures:||Gargoyle - Genestealer - Hormagaunt - Ripper - Termagaunt|
| Biovore - Hive Guard - Lictor - Neurothrope - Pyrovore |
Ravener - Tyrant Guard - Venomthrope - Zoanthrope
|Monstrous Creatures:|| Carnifex - Dimachaeron - Exocrine - Haruspex - Malanthrope |
Maleceptor - Mawloc - Tervigon - Toxicrene - Trygon - Tyrannofex
|Gargantuan Creatures:||Cerebore - Dactylis - Hierodule - Malefactor - Nautiloid - Viragon|
|Flying Creatures:||Harpy - Harridan - Hive Crone|
|Bio-Titans:||Dominatrix - Hierophant - Hydraphant - Viciator|
|Other Organisms:|| Bio-Weapons - Cortex Leech - Mieotic Spore - Mucolid Spore |
Mycetic Spore - Spore Mine - Sporocyst - Tyrannocyte - Zoats
|Members of the Genestealer Cult|
|Cult Leaders:||Broodlord - Genestealer Patriarch|
|Specialists:|| Biophagus - Clamavus - Jackal Alphus |
Locus - Magus - Primus - Nexos
|Cultists:|| Abominants - Atalan Jackal - Brood Brothers |
Genestealers - Genestealer Aberrants - Sanctus
|Hybrids:|| Acolyte Hybrids - Hybrid Metamorphs |
Neophyte Hybrids - Kelermorph Hybrids
|Familiars:||Alchemicus - Mindwyrm - Soulsight|
|Vehicles:|| Achilles Ridgerunner - Chimera - Dirtcycle - Goliath Truck |
Leman Russ - Sentinel - Tectonic Fragdrill - Wolfquad
|Playable Factions in Warhammer 40,000|
|Imperium:||AdMech:||Adeptus Mechanicus - Mechanicus Knights|
|Army:||Imperial Guard - Imperial Knights - Imperial Navy - Militarum Tempestus - Space Marines|
|Inquisition:||Inquisition - Sisters of Battle - Deathwatch - Grey Knights|
|Other:||Adeptus Custodes - Adeptus Ministorum - Death Cults - Officio Assassinorum - Sisters of Silence|
|Chaos:||Chaos Daemons - Chaos Space Marines - Lost and the Damned - Renegade Knights|
|Xenos:||Aeldari:||Dark Eldar - Eldar - Eldar Corsairs - Harlequins - Ynnari|
|Tyranids:||Genestealer Cults - Tyranids|
|Others:||Necrons - Orks - Tau|