The plague cult of Clan Fester, Clan Pestilens and other members of the Pestilent Brotherhood doesn't sit right with the religion of the Horned Rat, God of Ruin. The true agents of the Horned Rat, the Grey Seers, suspect the Brotherhood of being heretics secretly following Nurgle, the God of Diseases. Clues of this heresy are scattered through the lore. There were always hints in the Skaven army books of classic WHFB editions, but several supplements go further.

引用自 Warhammer Fantasy Battles: Lustria (2004)：

The Skaven of Clan Pestilens bear corruption and disease as a sign of their dedication to all that is foul. They have risen from the nameless band that penetrated the jungles of Lustria, the majority of its members succumbing to the tropical diseases running rampant through the Lizardmen's realm, until the last remaining Skaven sealed a pact with a new god. They embraced their own plagues and maladies as gifts, sicknesses so vile that they were able to shrug off the jungle-born plagues with ease.

引用自 Warhammer Fantasy Role Play: Lustria (2023)：

The Skaven are not diligent recorders of their own history, and secretive regarding what they do know. Scattered and inconsistent tales shared between the Plague Lords of Clan Pestilens suggest that when the final remaining ratmen lurched back into the caverns to regroup, they instead encountered a horror in the depths beneath Quetza. Inside a previously undiscovered, filth-ridden throne chamber rested a corpulent Daemon marked with the same weeping sores, foul stench, and bulbous lesions that mutilated their own bodies. It is said that the leader of these ratmen, desperate to save their vile hides, promised what remained of his clan to the Daemon and its master, who they proclaimed as the Horned One,, and embraced the plagues that had so thoroughly decimated him and his kinsmen. In that day, those Skaven truly became Clan Pestilens, bringers of plague, disease, and filth.

Who's this daemon and who does it serve?
The supplement introduces him as Uthl’kritchnaak, Exalted Herald of Nurgle. As a fluent Queekish speaker, it struck a pact with the leader of the skaven who was therefore renamed Nurglitch. This name isn't an unfortunate coincidence: it tells of the true allegiance of Nurglitch and at least the inner circle of Clan Pestilens. Nurglitch has since become the traditional title of the leader of Clan Pestilens.

This heresy is explicitely mentioned in another WFRP supplement.

引用自 Warhammer Fantasy Roleplay: Enemy Within Campaign - Volume 4: The Horned Rat Companion (2021)：

Many of the practices and rites utilised by Clan Pestilens are similar to those employed by followers of the Chaos God Nurgle, and rumours that inner echelons of the clan are followers of the Plague Lord rather than the Horned Rat are regularly circulated (not least by the Grey Seers). No hard evidence can ever be brought before the Council, however.

An example is arch-prelate Kanker Flett, leader of the coterie of Clan Pestilens plague monks in Under-Middenheim.

引用自 Warhammer Fantasy Role Play: Middenheim - City of the White Wolf (2021)：

Kanker and his entourage keep a low profile, muttering litanies of devotions to the Horned One to those with ears to listen (...). The Horned God he worships is Nurgle.

The Loathsome Ratmen and All Their Vile Kin (2004), the authority on Skaven lore, also establishes a parallel between human followers of Nurgle and the Clan Pestilens in their common use of "the insidious weapons of disease".

This heresy is carried out in gameplay:
- The first Nurglitch benefits from the special rule Scion of Corruption which makes him immune to spells from the lore of Nurgle. This is mentioned in Warhammer Fantasy Battles: Lustria (2004).
- The Liber Bubonicus, an ancient grimoire carried by Lord Skrolk, contains spells from both the Skaven lore of Plague and the Lore of Nurgle. This is mentioned in Warhammer Fantasy Role Play: Lustria (2023).
- The 4th edition book Warhammer Armies: Skaven (1993) goes further and states that Clan Pestilens members are immune "as are followers of Nurgle" to the effects of Nurgle spells cast with the Liber Bubonicus.

What do you think of this theory, man-things?