The Unmaker

Once a living being from our multiverse, the Unmaker ascended to the status of what we would consider a god. Sadly, it made him quite mad. Originally from a well-ordered reality, the Unmaker was a scientist who pursued secrets that were hidden from the lesser realities for a reason. Upon the day of his apotheosis, his body transmuted from his original form to a disgusting and viscous green sludge. His universe was destroyed near instantly, and other beings from outside our multiverse had to fling it into the Mad Realm to keep the infection from spreading.

Qualities
Non-cosmic creatures are revolted by the mere presence of this creature, even somehow across dimensional boundaries. This implies that the corruption staining the god was not created by it, but bestowed by an external power, since a way to gain dimensions has not been discovered or is completely impossible.

Allies
The Unmaker has cults throughout many universes, and his influence has even spread outside our Omniverse. Some higher beings clearly admire its devotion to destruction, and have granted it the ability to create avatars. Some species in other universes have discovered great masses of slime languishing in space, and to them, dreaming inscrutable dreams.