Having talked with Astrid you are sent to hide in the Night Mother's coffin in Cicero's room. Head down the slope to the main chamber of the Sanctuary. Go up the stairs and through through the open door. Open and enter the room behind the first door on the right.
Head to the Western side of the room. Go over to the coffin and pick the lock (Novice level). Step inside and then wait. It will not take Cireco long. It is clear he is addressing the Night Mother as you listen to the conversation. She will then start talking to the Dragonborn. She will dub you the listener and then explains that she is sending you to Volunruud to talk with Amaund Motierre. She will also tell you to give a phrase to Cicero: "Darkness rises when silence dies."
You will emerge from the coffin and face Cicero. At first it seems like he is about to attack and is talking with you. Then will can tell him the phrase. This will make him overjoyed that the Night Mother has finally chosen a Listener. Just after that though, Astrid will barge into the room. She demands whoever else is in there to reveal themselves. Here, Cicero reveals to her that the Dragonborn has been chosen to be the Listener. She will balk about being ousted, to a degree, from control of the Dark Brotherhood Family here. She will tell you to let things be for now and to head over to Nazir for another contract.
Make your way over to Nazir. He will offer you two contracts that need to be completed. Take them and the quest will finish.