Genestealer

Genestealers are a type of Tyranid that can exist away from the nurturing and controlling influence of the Hive Mind, using their own innate intelligence and brood telepathy to form tight-knit groups.

They can survive for decades or centuries on worlds, hiding their presence and infecting more and more of the population until the time to strike arrives usuallycoinciding with the aerial of the hive fleet and the wholesale invasion and consumption of the world itself.

In this way the Genestealers are scouts to the Tyranids at large and travel unseen on space ships to new worlds where they infect the population and create more genestealers that carry a part of the populations DNA in them. When the Hive fleet arrives this information is sent back to the Hive Ship which allows it to quickly adapt to the planets environment and create the correct organisms to conquer the world swiftly.

A purestrain Genestelter (created by the hive mind to start an infestation on a new world) is about six feet tall, has four arms ending in razor sharp claws that can cut through plasteel and is a match to even a Space Marine. They don't stop growing though and once an infestation has taken hold the starting Genestealer, ancient and powerful, is often referred to as a Brood-Lord.