For all we know Ketheric is just the Act 1 Boss Antagonist. No more special than... [DOS2 spoilers]
Alexander, the Sallow Man, the Shadow Prince, or, if you are being very generous, Adramahlihk
This is what I think too. He looks too obvious of a villain, sounds like a red herring to consider this guy as more that yet another puppet of the absolute. We have seen countless of those already.
