The Dark Brotherhood guild can be summarized in three words – Assassins For Hire. They carry out their daily blood shed int he name of Sithis. They are deadly mercenaries who are dark to the core – To join them, you will have to be sinful as they are.
Before joining Dark Brotherhood, you may be attacked by these Assassins – after killing them, loot their bodies, and you will most probably find a note stating ‘The Black Sacrament has been performed’ and that payment has been received for the contract.
You would be doing the same when you join this Guild so prepare yourself for bloodshed.
You will have to act like a Dark Brotherhood member to join this Guild. Head to Windhelm and get into the Arentino house which has a simple lock. Talk to Aventus, he will tell you to kill somebody in Riften’s Honorhall known as Grelod the Kind.
When you get there, enter into the orphanage. Grelod will be talking to some children, just kill her anyway you like as it doesn’t need to be clean. You can ask your follower also, once you are done, get back to Aventus.
Keep roaming in the streets of Riverwood until you get a letter from the brotherhood. Sometime later you’ll be kidnapped by Astrid often. She will ask you to kill somebody, its up to you…whomever you kill as she wants people dead (Alea, Vasha or Fultheim). When you’ve made her happy she will tell you the location of the Dark Brotherhood. It’s at the western side of Falkreath.
Go to the main entrance and choose the following answer “Silence My Brother”. Once you’re inside go and talk to Astrid, she’ll give you some new gear “Shrouded Light Armor”. Now that you have the gear, go to the North where you’ll meet a new family, just on the left side is a kill word of power.