This quest is crap - i have to spend eternity guarding nocturnals place after I die? In exchange for "power and knowledge from nocturnal" - none of which is given btw. The free armor looks nice, but it pales in comparisson to mine. I'm going to sovengard when I die - let mercer frey have it all.
I felt the same way and almost didn't go through with the quest, but I did some reading on reddit and other places, and it turns out that the daedric princes can't make any claim on your character's soul. Being Dragonborn means your soul is not human or elven, but is an aspect of Akatosh himself. Spiritually speaking, your soul outranks all the daedra. Once you die, you belong to Akatosh, period. The daedra can't do a thing about it.