[O'dyllita] Song of Tunta: 

Sylvia was once expelled by this unknown God of Depravity and Hadum was born
in her absence, killed the God of Depravity, and confined it in the Blackstar!

The God of Depravity is "Godr Ayed", god of humans and destructive light. So supposedly he is inside the blackstar, further confirmed by the description of the Fallen god armor pieces says it is that the "Fallen god", defeated by the "God of Despair" otherwise known as Hadum.

Lastly, during the blackstar questline one fights an "incarnation of corruption", which would mean that the one confined in the blackstar is actually Kzarka or one of his offspring.

So either Elion is Kzarka or something is weird.