This article is about the vanilla Morrowind faction. For the faction in Tribunal, see Tribunal:Dark Brotherhood.
The Dark Brotherhood is a guild of assassins that split away from the Morag Tong, and have been bitter rivals ever since. They are one of the unjoinable guilds in Morrowind, and are featured in the Morag Tong questline, where you will be tasked with preventing their expansion onto Vvardenfell. They have allied themselves with Dagonites, cultists of Mehrunes Dagon who provide them strongholds through Vvardenfell. The Dark Brotherhood had also stolen from the Morag Tong the Threads of the Webspinner, which can also be found carried by Dagonites.
Their leader on Vvardenfell is Night Mother Severa Magia, who can be found in Ald Sotha. Other members of the guild include 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, but they may also be the Dagonites that are allied with them. 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.
For historical information about the Dark Brotherhood, see the lore page.