The Supreme Tribunal was the chief governing body of the Galactic Empire. The Emperor, Cos Dashit, held the formal title of Consul to the Supreme Tribunal. It contained at least five members.
Other individual members of the Tribunal included Vantos Coll, Darth Vader, and presumably Crispin Hoedaack. These three formed a cabal intending to take control of the Empire following Hoedaack's successful annexation of the Aquilaean system.
The members of the Tribunal once represented the Great Families, but the cabal had already dislodged them from power.
Behind the scenes
In the rough draft, the Supreme Tribunal holds a similar position as the Imperial Senate in later versions. The elimination of the Great Families seems to have eventually become the dissolution of the Senate. The various members and their debates were transformed into the meeting of the Death Star's command staff.