In the mirror universe, the ISS Excelsior (NCC-2000) was a Terran Empire starship, the prototype Excelsior-class battleship in service to the Imperial Starfleet in the late 23rd century. (VOY - Obsidian Alliances novel: The Mirror-Scaled Serpent; TOS video game: Shattered Universe; TOS - The Mirror Universe Saga comic: "Behind Enemy Lines!")
- References from numerous sources establish that the various depictions of the mirror universe may not take place in concurrent timelines, meaning there could be a number of divergent alternate reality versions of this topic that do not share the same continuity. The references below this note are separated into subsections to minimize confusion regarding the divergences.
Behind enemy lines
In the year 2285, the Terran Empire had nearly finished construction of Excelsior, but were unable to make it operational. During their attempted invasion of the Federation, the Terrans were able to study the counterpart USS Excelsior and determine how to make their version operational. The plan failed, however, as Captain Scott had deliberately sabotaged the Excelsior to thwart the reverse engineering process. The Excelsior was placed under the command of Captain Blaine and led an Imperial Starfleet task force to find the USS Excelsior. The Terran Empire's Excelsior was unable to draw enough power and was easily defeated in battle. (TOS - The Mirror Universe Saga comic: "Behind Enemy Lines!")
In the late 23rd century, ISS Excelsior was under the command of Captain Styles. When Styles betrayed the Empire, Excelsior crewmember Tuvok was forced to kill Styles and take command. (VOY - Obsidian Alliances novel: The Mirror-Scaled Serpent)
By the early 2290s the Excelsior was commanded by Captain Hikaru Sulu. The crew of the ship exchanged places briefly with their counterparts from the primary universe after an incident at the Janus Vortex. After fighting against the ISS Enterprise-A under the command of Pavel Chekov, the crew from the primary universe attempted to reach Janus Ultima after the destruction of Janus Prime. Along the way, they came under attack from Klingon forces until they were driven off by the ISS Renegade who escorted the Excelsior back to friendly space without knowing that the Sulu in command was not from their universe. (TOS video game: Shattered Universe)