The Elder Scrolls III: Tribunal is an expansion pack for The Elder Scrolls III: Morrowind.
A quote from Bethesda Softworks:
In Tribunal, a ruthless new king has taken the throne of Morrowind, while an aging god drifts deeper into madness. Into this chaos you are thrust, with deadly assassins close behind you and Morrowind's uncertain future ahead. Your journey will lead you to Mournhold, capital city of Morrowind, to the Clockwork City of Sotha Sil, and through massive, epic-sized dungeons.