Genestealer
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...
Rogue Trader
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."
Space Hulk
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.
Later Editions
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 (fuck-ugly) child and revere the Genestealers either as gods or as creatures sent by gods (the exact interperation 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 sub-light speeds, 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 defences in preparation for the fleet. The fleet will consume the Cult along with everything else on the planet, but the Genestealers (and their hybrid servants) are just fine with that.
It's like they attempted to make the already grimdark-as-fuck Xenomorphs even more grimdark by crossing it over with The Shadow over Innsmouth.
Genestealer Variants
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 (Jesus Christ) that gives a squad of Genestealers the same unstable properties rule, but now lack the assaulting from Outflanking because, y'know, assaulting was SO FUCKING BROKEN and GeeDubs needed every assault unit to be shit.
On the Tabletop
In battle, both Ymgarl and Purestrain Genestealers are ridiculously deadly in melee, both in fluff and on the tabletop. On the tabletop, when a brood of Genestealers charges something, it's very unlikely to survive to the next turn unless it's a Land 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!
Non-human Hybrids
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. 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.). 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.
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.
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 empathic 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 fucking undead robots. If you even consider this possibility you're fucking retarded.
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. Brilliant!
Unofficial codexs
- Fly Lords of Terra Genestealer Apoc codex [1]
- So not only do genestealer cults have a codex now, but this man, Chris Showers, actually made a army of them! You'll have to Google Search them yourself, though, since the GamesWorkshop website *cough* [2] *cough* is slowly deleting anything that isn't the webstore
Gallery
See Also
- 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.