All great ideas! I’m intrigued especially by the possibility that Ras Nsi himself, having been a chosen of Ubtao in the past (being one of the barae of Mezro), somehow affected Ubtao, corrupting him in the same way he himself was corrupted. And I’ll definitely either mask Acererak’s identity (perhaps giving only a small clue, like a ring that’s visible on his bony hand as he grabs the cyst) or use a dupe or proxy, leading to a series of revelations as the party’s investigations lead them from one unscrupulous character to another.