Malvernis?

It seemed to have some sort of connection to the Gangue, the Inpure Spirit of the Stone that we encounter in DAO:

Dwarves believe that the Stone is not pure, for she bears a corruption as old as balance. For the dwarves to prosper the gangue — the waste and unstable rock — must be cut away from the raw Stone. Each dwarf must carve the worst of themselves away, but the Legion of the Dead bears a unique responsibility to protect the Stone herself from the darkness that afflicts her. The gangue can also be translated from Old Dwarven as "impure spirit-of-the-stone" and manifests as demons bound into the rock.


Malvernis seems to be an embodiment of this corruption, since the codex entries we come across seem to suggest that the dwarves imprisoned it on the surface because it threatened the purity of the Stone. The following terms are applied to him:

Pestilent One. Devoured things. Turning fair things into noxious waste. Consumed living, turning them to slive. Consumed bones. Foulness, poison, filth, desecration. Threatened the stone itself.


He would seem to be some sort of demon/abomination but the precise nature of this is unclear.   The being would also seem to have some sort of connection with both the Blight, which causes corruption of the living, and the Forgotten Ones, who the elves attribute with causing disease and death. According to the stories about Andruil, it would seem that the Void is also a place which corrupts those who go there, so again seems to have a connection with Malvernis.

It is anyone's guess whether this connection will ever be developed further or whether Malvernis is just one of those concepts that was introduced just for one DLC for variety and never heard of again.