Genestealer

Long before a hive fleet's shadow falls across a system, genestealers may already be there, having arrived generations earlier aboard a single drifting spore or a smuggled egg case no larger than a fist. They do not conquer through open war but through infection, implanting their genetic material into unwitting hosts and setting in motion a hybrid bloodline that spreads through a population like a patient, generational disease.

A world seeded this way often does not realize its danger until a cult has already taken root in its underhives, its industrial guilds, even its ruling council, all of them unknowingly or willingly serving an alien purpose planted in their blood. When the true genestealers finally emerge to fight directly, they do so with terrifying physical speed and four scything claws capable of parting ceramite plate, striking as the last, brutal phase of an infiltration that began in total silence.

Most disturbingly, genestealer cults can flourish and act entirely independently of any approaching hive fleet, sometimes for centuries, which has led more than one Imperial scholar to question whether the creatures are a weapon of the Hive Mind at all, or something older and stranger that the swarm has simply learned to make use of.