Wait for a few days and a messenger will bring you a letter with a Black Hand and the words "We Know." This will be your cue. Go to sleep anywhere. When you wake up, you will be in a room and you will have to kill one of three people. You can talk to them to find out which one you want to kill but it doesn't actually matter which you kill. After killing one, you will be given the password and a key to leave.
If you should choose to kill Astrid instead, this will begin the Destroy the Dark Brotherhood quest which will prevent you from ever joining the Brotherhood should you choose to later.