Cult of Amber
Having existed on the small world of Unctious for millennium, they came to believe that the snail's pace at which the Imperium progressed was still too fast. They instead embraced stagnancy, becoming determined to keep their great civilization the way it was for eternity. The cult's members included high-ranking politicians, PDF officers, and bureaucrats, allowing them to infiltrate the planet's power structure to pave way for the arrival of their god who would preserve the planet for all time, the great Lord of Amber. The Lord of Amber was, perhaps unknown to the cultists, Nurgle but nonetheless the cult grew more and more powerful over the millennia. Soon the Cult embraced not stagnancy, but rather regression and decay. Towards the end of M40 the Cult fell under the control of Magister Antonan Thrax, a high-ranking Administratum member. With access to both his cult's sorcery and the auguries of the Adeptus Astra Telepathica in the Sub-Sector, Thrax and his cult launched a coup on Unctious and created a massive dark ritual of conjunction that created the Warp Storm known as the Crow's Eye in order to finally pave way for the Lord of Amber.
The power of Nurgle did indeed manifest on Unctious, reducing the entire populous (including the Cult of Amber) down to their very bones. For his part in the plan Thrax was transformed into a Daemon Prince, Anathrax the Foetid. However due to the interference of Nurgle's rival god Tzeentch, the conjunction was ended prematurely and thus only Unctious was transformed into a realm of Nurgle.