tharizdun

after the gods discovered the existence of the shard of ultimate chaotic evil, they sought to destroy it, but could not succeed because its power exceeded their own.

tharizdun, originally the greatest of the gods and of lawful good alignment, removed the shard to the bottom-most depths of the elemental chaos, seeking to minimize its influence upon the plane. tharizdun, having possessed the shard, succumbed to its overwhelming influence and became more chaotic evil than even the greatest of demon princes. tharizdun now seeks nothing less than the utter destruction of everything.

all of the gods, both good and evil, sharing a fundamental interest in the preservation of the world and the planes, endeavored to vanquish tharizdun, but because of his own remarkable power, combined with some the shard's power that now resides within him, the gods could only succeed in banishing tharizdun to a potent demiplane of imprisonment.

io, who once was tharizdun's closest ally, was the god who ultimately performed the banishment. when io died, the precise knowledge of the demiplane of imprisonment died with him.