Abyss

Abyss is an unknown creature mentioned once in the writings of Khadgar.

Not much is known about Abyss, only that he is extremely powerful and somehow significant, Khadgar either doesn't know, or refuses to expose, anything more about Abyss.

During Khadgar's summary of Abyss, it is implied that he is a prominent figure among the Melkar, a faction mentioned often, but never explained in the writings of Khadgar.

"Of all of the sad and terrifying lays sung by A'dal, the most was, without doubt, that of Abyss, a prominent figure in the Melkar...""

Is speculated to be the tied with the Dark God, but it's only a rumor.