This Great Unclean One is one based on the parasitic Rafflesia flower. It’s name is Festus the Fragrant. My impromptu lore is that Nurgle created the flowers as an attempt to please Isha in his sort of way, as much as it was an attempt to spread rot to the Galaxy.

The so-called “Pestula Petals’” intoxicantingly putrid stench is known to induce a powerful stupor and to bring a sort of awe and euphoria to the Plaguebearers and Nurglings that flock about Festus.

The microscopic seeds of the flowers root into the bodies of trees, animals, and even the armor of tanks, its roots splitting bones, bark, and steel as it spreads, before finally bursting out as more odorously fragrant flowers. The flowers consume and integrate the genes of its hosts, allowing the flowers to adapt rapidly to even Astartes or Tyranid physiology.

Creatures that have been infested by the Pestula Petals often have their craniums ruptured by the growing flowers, turning their bodies and surroundings into smaller, but no less beautiful, versions of Nurgles’ blessed gardens.