Tribunal is the first official expansion for Morrowind. It adds the city of Mournhold (on Mainland Morrowind) to the game and a new line of quests.
The player, after installing the expansion, goes to sleep, but is awoken by a Dark Brotherhood assassin. After defeating the assassin, the player is prompted to make contact with Apelles Matius, a guard at Ebonheart. He in turn directs you to Asciene Rane, a mage who teleports you to Mournhold, a walled city on the mainland.
There, speaking to any Royal Guards will guide you to the sewers of the Great Bazaar, where the Brotherhood have set up their headquarters.
As well as an additional Main Quest, there are also numerous side quests for the player to complete, from acting in plays to fighting in an all-out brawl with a Bosmer.