Apostles of Contagion
Formerly of the Death Guard chaos marine legion, the Apostles of Contagion are a warband of demented Nurglites following their rotting master, the Chaos Lord Necrosius (an apothecary turned sorcerer), who believes that Grandfather Nurgle's gift of eternal life must be freely given to all in the galaxy; of course, this form of eternal life is as an unwashed zombie, but then what did you expect from a gang of deranged pusbags?
Having broken off from their former brothers, the Apostles follow only the will of the putrid god to whom they bend their arthritic knees. Because Nurgle's gift is for all to share, they don't care if they infect Imperials or their fellow chaos worshippers with the zombie plague, leading them to have quite a few enemies.
They bear a hatred for Typhus, the Herald of Nurgle, and his band of chaos marines, because Typhus initially betrayed the legion to Nurgle. Since the Apostles seem to enjoy their damnation now, it appears this hatred has evolved into the typical rivalry between champions of Chaos.
The Apostles were responsible for the widespread zombie plague outbreaks that occurred on Vraks Prime and so are partly responsible for the corruption and fall of that world.