The Dark Brotherhood, from the game Elder Scrolls IV: Oblivion (which is a kickass game), is an unofficial guild, or a grouping of people. These people are actually deadly assassins,who are hired by people all over the land of Cyrodiil. In order to make dealings with them, you must offer up someone's life, like maybe a family member, or someone close to you, which makes you decide if it's really worth it. The Dark Brotherhood tends to make their assassinations look as though they were accidents, but some people will suspect a lot of deaths to be their doing. In my opinion, they have the best quests and things to do.
-Did you hear about what happened to poor Baenlin? That minotaur head he had on a plaque on his wall dropped right on his head and killed him!
-Idiot, it wasn't any accident. It was the Dark Brotherhood, you should know that. Baenlin's nephew wanted that piece of estate to himself for a while now, so he requested Baenlin's death.