Myrkul's essence, not Cyric. If you devour him, you get a new ability, which is sort of a general avatar of destruction type thing. Everytime you hit an enemy in this form, you get some spirit energy. If you use that against Akachi, at some point you can devour him. This nets you immense power, Kelemvor kicks you out of the city, you wage a war against the gods, and eventually manage to kill a few before you are taken down yourself.
Of course, I could be wrong on a few points, haven't seen the ending myself.