In one of the houses a boy is praying to the night mother, and will mistake you for a DB assassin, kill the person he wants you to kill, you will be given a note from the DB, go to sleep in a bed, and when you wake up the quest will continue.
Last time(Oblivion) you had to kill someone innocent and take a nap after it. Then someone visits you to invite you into the dark brotherhood. Propably thats still the way to go?
In one of the houses a boy is praying to the night mother, and will mistake you for a DB assassin, kill the person he wants you to kill, you will be given a note from the DB, go to sleep in a bed, and when you wake up the quest will continue.