Enter the Sanctuary and try to help the survivors. Make your way upstairs and meet up with Nazir. Help kill his attackers and help him escape. Return to the Night Mother and climb into the coffin with her. When you get out of the coffin, you will be told that Astrid is still alive so return to Astrid's room and find her. Take the Blade of Woe and kill her if you like (you don't have to). Return to the Night Mother to end the quest.