The Gray Wasteland

While the existence of some planes are comprehensible based on the desires or allegiances of the beings living there, Hades, the Gray Wastes, presents a problem for planar philosophers. There are many theories, but most are some variant of the idea that the suffering of souls within the cosmos seeped into a reservoir or found its level. The existence of despair, in other words, created Hades. It is perhaps no accident that it exists in some metaphysical sense equidistance between the oppression of Hell and the malignant egotism of the Abyss.

The beings that willfully reside in the Gloom, both exploit and partake of despair. The devils hold the yugoloth were once a cadre of Hell, but deployment on the frontlines of the war with Chaos led to trauma. Their methods became first unsound and then alien. Devils will work with them to achieve goals, but hold them in disdain. 

Their primary value to Hell's high command is the process they have developed for extracting the essence of despair from souls of beings consigned to Hades. Over time, souls cease to fight against the pull of despair and are cover in gray dust or ash, like the victims of a volcanic disruption. Eventually their substance is wholly petrified to that of Hades, but before that point, there is a time where their souls are still somewhat fluid, yet tainted. The yugoloths tap the corpse and remove the fluid. It can be used to form the basis of an elixir that robs souls of their free will. The prospect of absolutely obedient masses greatly excites diabolic strategists, and they wish to study the substance to see if it can be produced elsewhere.