"Iä! Iä! Tyrannicus fhtagn!" - One of many unholy Genestealer Cult prayers
"Praise be! The star children deliver us!" - White Dwarf October 2016
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 Xenomorphs... IN SPAAA-wait a minute...
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 at close range, as each of their six strong limbs ended in sharp claws, and they had a gruesome lifestyle reminiscent of the Xenomorphs, but otherwise had little to distinguish themselves; 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 later, 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 an 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). 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 the host is either pregnant or has impregnated another member of their species and gives birth, their viciously malformed offspring creates a hive mind connection between itself, its parents and the Genestealers. 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.
They find the brood of Genestealers responsible and 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 and eventually form 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 and more "normal"-looking, but still obviously alien. That however 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 it becomes large 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 that has been massively retconned in the new Codex. (But actually, the piece of history present in the Index have both options: some cultists will desperatly fight back, will some others will accept their destiny with utter joy).
In older editions, they were just fine with it. In fact, it may even have been their end goal. 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. The 7th edition codex has pulled a 180 on this stance. Nowadays, 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 run home end up getting eaten by the very father they worshipped and the children they raised.
Why would GW do this? Perhaps because they hate the idea that anyone who is not of the Imperium or Chaos factions be allowed to have a happy ending - just look at them killing off the Tau's spiritual leader in Warzone Damocles, or how they decided to let the Deathwatch unwittingly botch Eldrad's plan to save the Eldar from Slaanesh, and weep. Or maybe they think that gratuitous drama and pathos are the only ways to maintain the Grimdark. Note they also didn't allow the Tyranids and Genestealer Cult armies to be actual Battle Brothers with each other despite allowing every single Imperial army to be Battle Brothers.
It's like they attempted to make the already grimdark Xenomorphs even more grimdark by crossing it over with The Shadow over Innsmouth.
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 the right rare circumstances. The new codex mentions 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 by the dude 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.
The Broodlord is an Alpha Genestealer, being connected to the Hive Mind as well as being a psyker (though he cannot use Warp Lance), thus allowing him to gain control over his burgeoning army of bastard children. The Broodlord's coming is often a time of great joy and celebration for the cult, as his coming is said to be a herald of the impending Hive Fleet invasion.
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 (for some unexplained reason) the fourth hybrid generation, whose children will be Purestrain Genestealers capable of "founding" new Genestealer Cults.
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 like the Broodlord he is much beloved by the other members of the cult.
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 pathetic. 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, an Ork Genestealer Hybrid or a Tau Genestealer Hybrid would look like is the subject of much speculation.
It should be noted that Orks reproduce asexually by way of fungal spores they secrete from their skin, so it's pretty hard to envision an Orkified Genestealer actually working, or lasting beyond the first generation or two, since they have no ways of mixing genes to produce purestrains in time (Orks might just spawn all the hybrid generations in a single sporing in no particular order, and those subsequent generations might do the same.). 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 (Actually, old fluff says that greenstealers tend to like orks more than tyranids, as in the tyranid hive mind, master of all genetics, is INCAPABLE of breeding out the orkiness, this leads to greenstealer cults actually fighting back or corrupting the tyranids who've come to nom them.) that or be envious of the guy with built in Choppas and still krump em to get a Dok to glue them on his own hands. Purestrain genestealers descended from Ork hybrids tend to have denser musculature (and most likely greater physical strength), which suggests that while not as prevalent as human hybrids, they're not exactly rare either. One 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.
Tau, similarly, have certain complications to the procedure -- not counting the fact their Kroot allies can taste if a person is infected or not simply by tasting some of their blood, or the fact tau-tech is probably advanced enough to scan for this threat once they know about it.
Firstly, tau are implied to have much lower sex drives than humans. Nope, Tau like it just as much as humans, it's just tempered by their authoritarian ideology when it comes to actually having kids. 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. According to the new Genestealer Cults Codex, not only can Tau-Genestealer hybrids happen, but 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.
While it probably is possible for Genestealers to infect the Eldar, it's unlikely that they'd really 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), 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). Thus it's going to stay as pretty much just a possibility for fa/tg/uys to wild mass guess over.
As for the Necrons, they're goddamn undead robots. If you even consider this possibility you're fucking retarded.
Dawn of War 2
Genestealers are pretty lore accurate in Dawn of War 2. Eats whatever they face. They are acting as the ultimate melee troops for Tyranids in tier 2. Lets make some count: 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 muderfuck 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. In the fluff, their claws are so damned sharp that they might as well be 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.
Genestealers have a squad leader upgrade, which the 5th edition changed from an HQ to a troop choice, called the 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 broodlord into rape machine in challenges, as he able to bring down almost any independent character in the game without eternal warrior, while costing much, MUCH less.
Also take note that genestealers do NOT like Hellhounds. They bring the smell of burning chitin into genestealer's breakfast. Avoid them at all costs. A Banewolf squadron will just make them shit out their guts under AP 3 fleshbane templates.
Thanks to 7th edition allowing "Come the Apocalypse" alliance this can be done once more! However, this is pretty much moot now that genestealers come with their own codex; aside from some extra vehicle choices, genestealer cults already come with their own human troops and tanks.
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 even then they're 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 guardsmen 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 Guardsman vehicles.
Genestealer Army list
For those of you who wish to field these in their entirety rather than through a Tyranid army, well then you are in luck. The Fly Lords of Terra have come up with a list that can be used in Apocalypse games. But since this army has an underdog feel to it, you could get away with using it in standard games as well. Not only that but the file provides the reader with a conversion ideas section: http://www.belloflostsouls.net/2016/02/40k-genestealer-cult-deathwatch-rule-coming.html
- 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|
|Cultists:|| Brood Brothers - Genestealers |
Genestealer Aberrants - Genestealer Familiars
Genestealer Magus - Genestealer Primus
|Hybrids:|| Acolyte Hybrids - Hybrid Metamorphs |
|Vehicles:||Chimera - Goliath Truck - Leman Russ - Sentinel|
|Playable Factions in Warhammer 40,000|
|Imperium:||Adeptus Custodes - Adeptus Mechanicus - Imperial Guard - Imperial Knights - Space Marines|
|Officio Assassinorum - Inquisition - Deathwatch - Grey Knights - Sisters of Battle|
|Sisters of Silence - Squats - Militarum Tempestus|
|Chaos:||Chaos Daemons - Chaos Space Marines - Lost and the Damned - Renegade Knights|
|Xenos:||Dark Eldar - Eldar - Eldar Corsairs - Harlequins - Ynnari|
|Genestealer Cults - Tyranids|
|Necrons - Orks - Tau|