While on your travels in Skyrim, go to any inn and ask for the latest
gossip. The innkeeper will tell you that there is a child trying to
perform the Black Sacrement in Windhelm. If you happen to be in
Riften, speak to a man named Maul. He will be standing against a post
as soon as you enter the city. After sharing some tough words with
you, he gives you information on both the Thieves Guild, and the Dark
Brotherhood; for a price, of course. You could alternatively talk to
Idesa Sadri inside Candalhearth Hall in Windhelm and ask about a
cursed child performing the Black Sacrement. Idesa can also be
overheard talking to Grimvar Cruel-Sea about the child, Aventus
Aretino, outside the Aretino Residence. Aventus Aretino can be found
inside the Aretino Residence, which has a novice-level lock. The child
will offer you the Innocence Lost quest, which will lead to the quest
With Friends Like These....
After Innocence Lost has been completed, a courier will appear and
give you a note with the imprint of a black hand and the words "We
Know" written beneath it. If you do not receive the note, wait for a
couple of days. Once you receive the note, sleep in any bed.
Thereafter, you are abducted by the Dark Brotherhood, taken to an
Abandoned Shack, and asked to kill one of three people in the room
(With Friends Like These... ). Killing one or all of the potential
victims will earn you an invitation to join the Dark Brotherhood. You
may also take the opportunity to destroy the organization by attacking
the Dark Brotherhood representative, Astrid. This will initiate the
quest Destroy the Dark Brotherhood!.
Best Answer
Have you already talked to Aventus Aretino? If you haven't go talk to him and complete the job. If you go to sleep afterwards you'll wake up in the abandoned shack in the swamp near solitude.