Taint

Forbidden magic. Inhuman blood. Elimination of these corruptions is the founding principle of the Imperium. Keeping humanity free of the Taint is the tenet of the Inquisition and Inquisitors will stop at nothing to eliminate its presence once found.
Though some are born Tainted as a consequence of corrupt bloodlines, others stumble into damnation through the forbidden arcane arts of blood magic, conjuration, and necromancy. Still others truck with beings from the outer realms, worshipping old gods or simply seeking to barter for their power.
However it happens, the Tainted inevitably invite disaster onto themselves and those around them.