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.
Rogue Trader
In 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
Genestealers received a significant bump in their threat level with the release of the Space Hulk board game. 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 their "totally-not-Xenomorphs" theme, 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.
Space Hulk-era Genestealers were re-designated "Purestrain Genestealers," created by the Hive Fleets themselves. They 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 insert a "seed" into a host's body. The seed alters the host so that it loses all memory of the infection and becomes subservient to the Purestrain's brood. The host is also driven to have children and to look after them; even though these children will be part-human monsters, the brood's psychic network will enforce a slavish devotion on the host's part.
As the family grows, they will isolate themselves from the normal humans so as to avoid discovery, founding a Genestealer Cult. 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 defenses 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.
Genestealer Variants
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 descendents will be more like that species, until (for some un-explained reason) 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 tentacular, 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.
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 genestealers' potency in melee is compensated, at least when they could cheese through infiltrate and outflank, by their lack of any shooting ability, weaker armor save, and their cost that meant that a tyranid player couldn't let them die like flies like they could other troop choices The fact they are faster than Eldars 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 frist 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 a broodlord, which formerly had a major role due to the biomorphs it could use, but being reduced to a troop choice basically made it just adding an extra punch, though it still has a better statline Space Marine HQs, except the 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.
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 Tau might be rarer as kroot can sniff out genestealer (or at least taste it). Also highly controlled police state dictate their "Greater Good" system makes hiding Genestealer cult much more difficult.
While it probably is possible for Genestealers to infect the Eldar, it's unlikely that they'd really get anywhere with any of the three varieties due to their highly advanced medical technology (all three, yes even the Exodites), universal empathic abilities (mainly the Craftworld Eldar, but the Dark and Exodite varieties have it to some extent), very regimented lifestyles (Craftworld), and the fact that irregular psychic activity amongst them gets located and dealt with very quickly (all three, 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, 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!
- 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! [2]
