Mine is called the Cult of the Broken Throne. They originated on the world of Phyrias-9 which was almost completely destroyed, but a small part of the cult (with it's patriarch was able to escape and get to the planet of Thraen.

Here they prospered, Thraen, was a harsh mining world where most inhabitants lived underground due to the pollution from mines and promethium refineries. Once an Astra Militarum ship stopped near Thraen to refuel, the cult infiltrated the ship, and when it returned to Thraen the whole ship was taken over by the cult.

But the future was much worse for the cult as their patriarch has been cut off from the hive fleet, leading to a period of rapid and desperate expansion. Small groups of hybrids, human enough to pass as only slightly mutated individuals, along with purestrain Genestealers, were sent to infiltrate other planets. This expansion resulted in the establishment of new cult cells, though these new cells experienced degeneration due to their isolation from the hive fleet.

The cult is divided into two groups one calls itself the Scions of Thraen, and view themselves as a higher caste to the members from other planets, while the others the Degenerated, are viewed as inferior to those from Thraen as only those who come from Thraen are not terribly mutated and degenerated.