You have to kill the emperor. Its a pretty fun quest, and if I can remember it correctly, the emperor has a decoy that you kill and then you are sent out to kill the real emperor. You come back to the sanctuary where it is under attack. You go through and have to save that one redguard from dying, and then you climb into the Night Mothers tomb/coffin/thing to survive an explosion. Once you get out, the fires are out and the only two people who havent died were the redguard and that evil vampire girl. Then, the night mother tells you that the chick who runs the dark brotherhood preformed the black sacrament on herself. You then come in and kill her. Then, you move to the sanctuary in Dawnstar, where you previously killed that annoying jester. You are now the leader of the dark brotherhood and can fix the place up.
Spoiler alert.
1. i already beat the quest
2. youre an ass
3. people have been banned for putting "spoiler alert" after the spoiler before