Dark Brotherhood note
The Dark Brotherhood is a group of assassins who are willing to assassinate virtually anyone as long as they are paid enough for the service. They are a perverted offshoot of the "legal" assassins guild, the Morag Tong, who are based in Morrowind. While the Morag Tong are a respected, if feared presence in Morrowind society, the Dark Brotherhood are a secretive underground cult, the members of which worship Sithis, the avatar of death.
Characteristics
It has been revealed that they can be joined in Skyrim. You will be notified if you are eligible to join when you receive a slip of paper with a black hand on it, on the back of the paper will be written, "We know" This note may be recieved upon murdering someone in game similar to Oblivion's. The "We Know" signifies that they are aware of your actions, and may be eligable to join the Brotherhood.
Joining the Dark Brotherhood
While on your travels in Skyrim you will be approached by a small child, this child will give you a strange and disturbing quest in The Rift, where you have to go and kill a woman at an orphanage who treats the children badly. Upon killing her the children will see and start laughing and thanking you for killing her; one will even praise the Dark Brotherhood. Eventually this will get you noticed by the Brotherhood and will likely get you into The Dark Brotherhood.
Dark Brotherhood Shrine in Skyrim