The Dark Brotherhood is a guild of assassins shrouded in shadow and mystery which has been active all over Tamriel. While their business is certainly not legal, their existence has typically been tolerated or ignored. Whenever an individual with legal authority takes an interest in their dealings, the Brotherhood relies upon a combination of bribery, blackmail, coercion, and murder to keep their activities hidden. They have been bitter rivals of the Morag Tong since the Second Era. The Brotherhood is also known for their worship of the Dread Father Sithis. Dark Sisters are accepted into the Brotherhood as equally as Dark Brothers, though members of either gender are often referred to as Brothers.
The Dark Brotherhood is one of the unjoinable guilds in Morrowind. They are based in Mournhold and you will encounter them in the main quest of the Tribunal expansion. See the Tribunal article for more specific information about their role in the expansion. They have a considerably greater role in Daggerfall, Oblivion and Skyrim.