Your first point of call will be to visit any inn within your vicinity and ask the innkeeper for gossip.
The innkeeper will speak of a boy from Windhelm, named Aventus Aretino, who is attempting to perform a Dark Brotherhood ritual known as the Black Sacrament.
Speaking to a man named Maul in Riften, located in Mistveil Keep, will also give you information about the Dark Brotherhood for a price.
Alternatively, you can also speak with the children at the Honorhall Orphanage in Riften, who will give you the same information.
Both options will prompt you to go and find the boy within a new objective. Aventus can be located at his parents’ home in Windhelm and speaking with him will trigger a new quest.
Innocence Lost quest
The first step of the quest will be to return to the Riften orphanage, if you took that option before, to carry out the murder of Grelod the Kind without bounty.
You will then have to allow around 24 hours to pass before the Brotherhood contacts you. This can be achieved by entering a city and waiting for a courier to pass you a note, written “We Know” with a black hand printed.
You will then have to put yourself in a vulnerable position and sleeping for a period of time is recommended to allow you to be abducted.
Upon awakening in an abandoned shack, you’ll be confronted by Astrid, the Dark Brotherhood assassin. You will then be given the option to begin the Friends Like These quest, requiring you to kill one of three victims in the room, thus earning an invitation to join the Brotherhood.
Alternatively, you can also opt to destroy the Dark Brotherhood by killing Astrid, which will trigger the Destroy the Dark Brotherhood! quest. This will be your only chance in the game to defeat the Brotherhood.