Ygilthek

Ygilthek The Maw is the Lesser being from the Abyss that is controlling bluejacket. He seeks to gain the Winterthorn and use it to free the Dissident and kill the Paragon, thus plunging the planes into true madness and chaos.

He appears as a great, sickly green oozing mass, with eyes and teeth and limbs and tendrils sprouting and disappearing out of his murky surface. Until he confronts the party, he is very much out of sight. He is constantly using his tendrils to control the strings puppeteering the Purple Army Fighters. If one were to follow the strings from a fighter, it would eventually lead to him.

During the final confrontation with Bluejacket once he gets the winterthorn, the Serene will guide the fighter attacking to cut the strings instead of bluejacket himself, giving the player truesight and blessing their weapon. Upon realising that he's lost bluejacket, and that he is visible with True Sight, Ygilthek will become furious, knocking bluejacket aside and proclaiming that it doesn't matter what they do, he has the Winterthorn now, and they're not powerful enough to stop him.

The best ending would be for Bluejacket to regain his sense and use the Winterthorn to kill him, by changing the sword of ice into a lance of radiance, which will instantly kill him.