Fight your way through the Penitus Oculatus Agents that are in front of the Dark Brotherhood Sanctuary. Expect a fair variety in them. You will find most of the Brotherhood dead. Festus Krex pinned to a tree outside the Sanctuary with a number of arrows. Just inside is a pair of Penitus Oculatus Agents talking about the Traitor. Just beyond them is the corpse of Veezara. Eliminate them. The rooms here are empty (unless you have yet to grab the Unusual Gem from Astrid's room) so head deeper into the Sanctuary.
In this room you will find Arnbjorn in his werewolf form fighting Agents. He will die even if you kill the Agents attacking him. Nearby you will also find the corpses of Gabriella and their pet spider: Lis. Continue in deeper after that by turn to the left and heading through the break in the flames.
Make your way deeper into the Sanctuary. You are looking for Nazir. Go through the room to the North and up the ramp in the next room. You will find Nazir fighting Arcturus. Having defended Nazir, turn and talk with him. Now it will be time to search for an exit.
As you are running and searching, you will hear from the Night Mother. Follow Nazir until you are nearby the Night Mother's Room. Once there, head over to her coffin. Open it up and go inside once again. You will hear her tell you to sleep as you can hear rubble coming down and some explosions.
Nazir and Babette will unearth the coffin and open it up to let you out. Just before they do though, the Night Mother will tell you to seek out Astrid here in the Dark Brotherhood Sanctuary. Turn to the South and go up the first Staircase on the left.
In the next room, turn to the right. You will see a bookcase has been moved. Head through the revealed passage. This lead to a formerly hidden room. You will find Astrid here, dying and badly burned.
Astrid will explain that part of it was to ensure the safety of the Dark Brotherhood. She will explain that she performed the Black Sacrament on herself to call you. When you appeared, it proved the power of the Night Mother beyond any doubt. She will tell you to take the Blade of Woe as your mark of Leadership. She will then tell you to kill her. You can equip the Blade of Woe and finish her with it. Any targeted effect will do the trick at this point on Astrid.
Now, return to the Night Mother. She will comment on Astrid and then explain that you still need to kill the Emperor. She will explain to meet Amaund Motierre in Whiterun at the [[Skyrim: The Bannered Mare| to learn the true location of the Emperor within Skyrim itself. It is time to finish this contract.