You have returned to the Dark Brotherhood headquarters to find it being raided by Commander Maro’s soldiers.
Four of them are standing outside the entrance as you approach. Sneak or fight past them and go inside. Sneak or fight your way through the complex to the room where you normally meet Astrid. If you approach quietly, you will overhear some of the soldiers talking about spreading fire throughout the place and how one of their number by the name of Arcturus led the rest of their men deeper down.
Fight your way further down and you’ll soon arrive to see the last moments of Arnbjorn’s life; he is taken down by archers while battling in his werewolf form. Quickly make your way to the dining hall, where Nazir is trying to fight off Arcturus and more of the soldiers. Move quickly to help him dispatch the attackers, then tell Nazir about the setup. There is not much time to talk; you need to get out of here before you burn to death.
Follow Nazir to the southeast, into the connecting corridor above the stained-glass window. You will then hear the Night Mother speak to you inside your head; she tells you to embrace her, as she is your only salvation. Go the Night Mother’s chamber, open her coffin, step inside, and fall asleep. Nazir causes the coffin to fall through the window.
You survive inside the coffin, and Nazir also had a way out. He and Babette are moving the coffin into an upright position. Just before the lid is opened, the Night Mother speaks to you again and tells you to speak with Astrid, here in the Sanctuary.
Step out of the coffin, and after an exchange with Nazir, move out of the grotto and up the stone stairs to the south, past what remains of the entrance chamber. Turn right (west) and head north to find Astrid, who is lying on the floor, burnt beyond recognition, moments away from death, surrounded by the pattern of lit candles used in the Black Sacrament.
With her last words, Astrid confesses that she betrayed you to the Penitus Oculatus in return for a promise from them to take no further action against the Dark Brotherhood. Ske now realises that what she did was an unforgivable mistake and that the Dread Lord, Sithis shall judge her accordingly. She enacted a Black Sacrament and prayed for an assassination contract. She says that there is still a chance you can rebuild and begin again. You lead the Brotherhood now. Her Blade of Woe is yours, to perform the transfer of power, which is achieved by killing her.
Do as Astrid says, end her life. After it’s done, the title of leader of the Dark Brotherhood and the Blade of Woe are yours. Return to the Night Mother’s coffin. Your task of rebuilding the Dark Brotherhood begins now.