Genestealer

In the fictional universe of Warhammer 40,000 by Games Workshop, a Genestealer (scientific name "Corporaptor Hominii") is an alien creature associated with the Tyranids. "Genestealer" is also the name of the second boxed expansion for the 1st edition of the board game Space Hulk, also by Games Workshop.

Genestealers appeared in the first edition of Warhammer 40,000 (Rogue Trader), but at the time were not related to Tyranids. They were simply noted as an "enigmatic monster ... from one of the moons of Ymgarl", now "spread throughout space" and "threatening to become a real menace". Since then it has been revealed that they are the advance forces of a Tyranid invasion and are now often seen as part of a Tyranid army in the Warhammer 40,000 game.

Reproduction and Genestealer "Cult"
Genestealers reproduce by introducing their genetic material into a host from another species; this is normally a human, but can theoretically be any humanoid race (which would include Eldar or Orks). The Genestealers have no genders, and require a creature of any species, any gender. The Genestealer will find a suitable host and hypnotise them using its unusual, alien eyes. The Genestealer then thrusts its long, whip-like tongue into the body of the host where it deposits its genetic material in the form of an egg-capsule, which grows quickly into an immature Genestealer/Host Hybrid, which then bursts out of the host, obviously killing it.

Interestingly enough, newly born creatures, although fundamentally Genestealers, will have characteristics inherited from the host parent. Thus a Genestealer/human may have a vaguely humanoid head, or only two arms instead of the usual four, and perhaps its tail will be shortened or missing. A Genestealer of four or more generations consistent human parentage would pass for a human on cursory inspection, although a closer look would reveal a bluish skin, sharp pointed teeth and rather disturbing stare. On their home world, Genestealers exploit a large leech-like creature called a Csith which is the usual mate. Matings with a Csith always produce natural Genestealers, no matter what its parentage otherwise.

All of the members of this unusual "family" of hosts, Hybrids and Genestealers become psychically linked to each other, and become fiercely loyal to their "family". Over time, the "family" grows as its members continue to breed and bring in more members, and the "family" will eventually grow large and become a Genestealer cult, sometimes worshipping some vaguely defined entity which will 'deliver' the worshippers and their planet from the harshness of life in the Warhammer 40,000 universe.

The psychic link maintained by the "family" or the cult - henceforth "brood" - acts as a psychic beacon, attracting the forces of the Tyranids to their planet. As more members are brought into the group, the psychic beacon grows stronger. As shown with the events leading up to the Tyranid invasion of Ichar IV (Kelly and Chambers, 2004), a brood can eventually grow so large that it will eventually begin conducting sabotage or causing riots. Broods have even been observed rebelling against Imperial authority, attempting to seize ruling power over the planet for themselves. The ultimate goal (whether the brood knows it or not) is to sow discord or even neutralize planetary defenses in order to ease the Tyranid conquest and consumption of the planet.

Although Genestealers are frequently born from other species, they are said to lack creative intelligence (Priestley and Chambers, 1993). They are unable to wield weapons, even those of the species from which they are born (for example, a Genestealer born from humans could not wield guns or swords); Hybrids have some degree of intelligence and are able to make use of weapons. Genestealers do have incredible animal cunning, however - they have even been seen acting as a sort of scout for Tyranid armies, stalking ahead of the main force.