Benzegegoth, shortly after he is unshackled. Perhaps he would have a different form if he was unshackled longer.
Benzegegoth is a spirit of the Shadowfel who was previously enslaved by Swayne in a spirit shackle, until the party disrupted the shackle on behalf of the Rahjs. He may or may not have since been re-shackled. While shackled, he gives his shackler control over a shadow monster. Swayne had originally been using this monster against a troublesome Drow governor, but on the advice of the Mindhunters, he instead sent it against Alyssa, despite the fact that it is useless against the undead. Their reasons for this (like most things about Mindhunters) remain unknown.