Where the Chaos Thing Fell

When the hordes of the Abyss surged toward the very borders of Hell, one of the mightest of that host was only brought down on the plains of Gehenna. Where the great worm fell, it created a gigantic crater, contributing to the broken nature of the plane to this day. It's in this crater that the corpse of the creature remains.

The shadow of its bulk is tangible, like a black, velvet fungus, it moves over time as if chased by a sun that Gehenna does not have. It is not good to touch the shadow, as it will grow on anything until it consumes it. The Ultroloths sacrificed any number of souls and simulacra in their experiments trying to find a way to bend it to their purposes but to no avail.

They found no use for the shadow, but the same can not be said for the carcass. The Yugoloth consider it a goldmine. The crater is held in the highest security; not even their diabolic allies and clients are allowed to visit their mining and rendering facilities. The dissolution of an abyssal monstrosity is not like the decay of some corpse on the Prime Material Plane. Freed of the monster's alien, but dominating sense of self, its flesh slowly sloughs free and becomes all sorts of smaller grotesqueries. The Ultroloth sorcerer-scientists have been ingenious in the applications they have found for these creatures, including using them as a substrate for the generation of new, lesser Yugoloth. The things also found their way into weapons and material for armor. 

The plague caused in Hell by an attempt to use the creatures' ichor as an enhancement for soldiers was, at best, a minor setback.