Grand Admiral Garth of Izar

In the mirror universe, Kelvar Leonard Garth (2208-2269), better known as Garth of Izar, was the Grand Admiral of the Imperial Starfleet of the Terran Empire in 2269. His victory at the Battle of Axanar was one of the most famous in the history of the Terran Empire.
Following Captain Spock's successful takeover of the ISS Enterprise NCC-1701 in 2267, Garth was one of many officers to send him a congratulatory message. In 2269, Empress Hoshi Sato III ordered him to promote Spock to admiral in recognition of his actions in the Tellun system, destroying the rampaging M-5 computer and the capture of a Romulan Bird-of-Prey.
Inviting Spock to a lavish ceremony in his honor on Elba II, Garth correctly deduced that his rise to power was the result of an alien weapon which he had taken from his predecessor Captain James T. Kirk. Using the shape-shifting techniques that he had learned on Antos IV many years before, Garth hoped to beam onto the Enterprise in Spock's form and claim the weapon for himself. He would then use it to kill Spock and his wife Commander Marlena Moreau before turning it against the Empress and declaring himself Emperor.
However, Spock - having learned of Garth's ability from Kirk's records - anticipated this move, developing a chess-related code, which foiled Garth's attempt. Infuriated, Garth opened a channel to his flagship, the ISS Imperious, to destroy the Enterprise. However, Marlena killed him using the Tantalus field before he could give that order. His Orion mistress Marta suffered the same fate almost immediately afterwards.
Garth's successor as Grand Admiral was Matthew Decker of the ISS Constellation NCC-1017, who served in that position until being killed by Spock in 2277.