To trigger the alternate ending, when Molag asks you to kill the priest with the rusty mace, drop it instead. In the vanilla game, this is a hard quest to RP properly if you are a good character, because there's no justification for why you'd HAVE to kill the priest of Bolethiah (unlike the poor Vigilant). This adds an option to spare the captive priest of Boethiah during Molag Bal's daedric quest instead of killing him.
