Converted from Dawn of War 2.
Genestealers infiltrate worlds on the way of the Great Devourer and infect imperial citizens, who start worhiping "Four-Handed Emperor" aftewards. This sects, called Genestealer Cults, spread in the underhive and rebel against Imperial authority. The same Cults serves as a beacon to Hive Mind, attracting Tyranid hordes. In the end, Imperial Worlds, torn by rebelion, become easy prey to ever hungry Tyranid swarm.
On their own Genestealers are fearsom fighters - they compete in swiftness even with Eldar or Imperial Assassins and their rending claws are sharp enough to tear through Terminator Armor.
Dawn of War Conversion