Pale Reapers

The Pale Reapers, formerly the Ivory Angels, were once a chapter of Space Marines serving the Inquisition in containing and sterilizing infectious threats that could potentially quietly infect and spread through the larger Imperium of Man.

This changed when Nurgle cursed the chapter with a mysterious ailment, the Vein Worms, that filled the blood of the chapter with writhing parasites, which formed a lattice of mutant, annelid flesh within the victim and eventually left them a dry, bloodless corpse.

Tracking the source of the ailment to the Writhing World within the Screaming Vortex, the Ivory Angels deserted the Inquisition to make a doomed crusade into the Warp Storm, at least partially because they couldn't stand the idea of a plague that had outperformed them.

When they finally reached the Writhing World, Nurgle's trap closed as warbands of Death Guard, mutants, daemons and other abominations rose up, crushing the already weakened chapter, and forcing them into a last stand. The survivors betrayed their loyalist brothers, finally cracking beneath the pressure and believing wholeheartedly in the inevitable transience of beauty and honor, and vowing to aid the defiling of such concepts wherever they could. They took the monstrous annelids of the Writhing World into their own flesh, becoming one with the undulating strings of flesh and being reborn as the Pale Reapers, walking, bloodless hives of parasites.

Organization
Overall, nothing leads the Pale Reapers beyond the hunger of the Worms and their gnawing urge to defile all that is beautiful and healthy.

That said, Apothecaries, of which the Ivory Angels unsurprisingly had in abundance became a highly regarded sect within the warband, creating newer and more deadly and infectious bioweapons, as well as horrifying, worm-filled monstrosities to aid the warband when it marches on its foes.