This article is about the vanilla Morrowind faction. For the faction in Tribunal, see Dark Brotherhood.
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 (personified chaos, also known as the Void). Dark Sisters are accepted into the Brotherhood as equals of Dark Brothers (though members of either gender are often referred to as Brothers).
Their leader on Vvardenfell is Night Mother Severa Magia, who can be found in Ald Sotha. In broader game lore, they are primarily based on the continent, to the south of Vvardenfell. "Night Mother" appears to be a Guildmaster-equivalent title, derived from the Night Mother, an eternal spirit that is the feminine counterpart to Sithis.
Other members of the guild include Carecalmo, Durus Marius, Movis Darys, Hrordis, and Tsrazami. It is also likely that anyone accompanying some of these people in their various hideouts are members as well. A non-member who has business dealings with them is Miun-Gei.
The Brotherhood also features in the Tribunal expansion, where you will encounter them on Vvardenfell and in ruins under Mournhold as part of the expansion's Main Quest. See the Tribunal:Dark Brotherhood article for more specific information about their role in the expansion. Since the Brotherhood does not technically exist as a faction in the game code without Tribunal, none of the vanilla Morrowind NPCs associated with the Brotherhood are listed as members of it in the CS (no one is at all other than Dandras Vules), and estimating their relative ranks within the organization would be conjecture.