Grunker
RPG Codex Ghost
Speaking of Unfair, just killed this bastard. Tough fight. He kept ignoring my summons and killing my mounts. Without Lunge and selective Sirocco, I wouldn't have won this fight.
Did you manage to negate his Concealment? I did it "clean" and without Sirocco and shit like that, but as you saw, it came down to the wire, and Concealment is DEFINITELY bugged for me. I have a save just before Playful Darkness and his Concealment cannot be countered in any of the usual ways - True Seeing and See Invis included. I haven't tested Faerie Fire.
This is the case for most enemies in my game by the way, and seems widely reported on reddit too. It makes the game mucho mucho harder.