Genestealer Invasion Force
Genestealers are a parasite race, and will die if they do not constantly infect new races. Purestrain Genestealers are immortal. The human brood brothers and hybrids are nevertheless not. Journeys through deep space using Space Hulks can take centuries or millennia, by then only the pure genestealers will have remained, the hybrids and brood brothers dying off long before they have reached their uncertain destination. Thus, some of these invasion forces consist exclusively of purestrains. However, sometimes the hybrids build special cryogenic suspension chambers for themselves and their purestrain brothers to endure the time in it.
Space Hulks are ancient and many contain lost technology from the Dark Age of Technology. For this reason they are highly valued by the Imperium, even though they may harbour genestealers and other threats. If research troops or pirates search a Space Hulk for technological artefacts, they often find themselves attacked by Genestealers, and then infected and often accompanied back to their home planet. Also, by landing or rescue capsules, the invaders can reach a planet where they attack mostly remote settlements, caravans, patrols or isolated garrisons to find the first hosts and to found a new Genestealer Cult.
In White Dwarf 115 an army list appeared for Genestealer Invasion Forces for the first edition of Warhammer 40,000. In the Codex: Tyranids (2nd Edition), the inclined player was able on account of the variable army arrangement to put up similar forces. Besides, the Citadel Journal 41 contains a suitable list for the third edition.