The Nameless One
It dwells to the south in the Green Hell that encircles the equator. It is one of the last demon lords of the old world, and likely once an Other that was corrupted by Chaos into an infernal creature.
The thorns unknowingly do his bidding in their attempts to erase all trace of civilization and worlds other than Lir. His seeds, such as Jivven, are bearers of a plague that causes flesh to bloom into fungus that eventually makes fertile ground for the spread of his great garden