This quest begins immediately after “Following a Lead”. Simply talk to Arquen between midnight and 3:00 AM at the farm at Applewatch and she will transport all the members of the Black Hand to the statue of the Lucky Old Lady in Bravil, which leads to crypt of the Nightmother. Enter the crypt and the traitor will reveal himself when the Nightmother appears. Defend the Nightmother, kill the traitor, and she will talk to you and promote you to the Listener of the Dark Brotherhood. The achievement will unlock when the initial part of the conversation with the Nightmother is complete.