Tharizdun

Seriously wounded Ioun during a struggle after the Calamity.
From Brom Goldhand: Tharizdun is also known as "The Chained Oblivion" Very evil god. Sealed, chained away.

I have a pretty big feeling / instinct that Tharizdun is either Vecna, The Death god that Raven Queen usurped, or both (I think Vecna may be the original death god).
Got even more about this theory in Vasselheim, need to split it off at some point.