The player is attacked by an assassin, later revealed to be a member of the Dark Brotherhood, an assassin's guild that spans Tamriel. To find out more about the Dark Brotherhood, the player is sent to Mournhold, the capital of Morrowind. Once in Mournhold, the player locates the head of the Dark Brotherhood and completes a series of side quests for the new King Helseth, and the Living God Almalexia.
The main additions to Tribunal are the inclusion of a new city, Mournhold, to the original game. Mournhold is not a contiguous part of the game world in Morrowind, and players are required to access the area through a teleportation service, with certain features such as levitation disabled to conceal the separation from the game world. Access to the content in Tribunal is possible after the player first goes to sleep, which will trigger the initial quest leading the player to be teleported to Mournhold.
Source: Wikipedia, The Elder Scrolls III: Tribunal , available under the CC-BY-SA License.