In the mirror universe, Marta was an Orion female who lived during the 23rd century. She was the "prime consort" of Garth of Izar, the Grand Admiral of the Imperial Starfleet. In 2269, Admiral Spock was introduced to Marta at a ceremony held in his honour in the Governor's Mansion on Elba II. Spock noted that her grin "hinted at the madness lurking behind her eyes."
As his concubine, Marta was aware of not only Garth's shapeshifting abilities, which he had learned on Antos IV many years earlier, but also his plot to assassinate Spock. Having correctly deduced that Spock's rise to power was the result of an alien weapon which he had taken from his predecessor Captain James T. Kirk, 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 Empire.
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. Marta was killed in the same manner immediately afterwards. (TOS - Mirror Universe novel: The Sorrows of Empire)