Garth of Izar was a decorated, highly-regarded officer in the Federation Starfleet, who rose to rank of Fleet Captain in the course of his service through the mid-23rd century. His career dissolved in a dramatic descent into madness and attempted genocide, resulting in his commitment to the Elba II asylum for the criminally insane.
Starfleet's fallen hero
Garth emerged from Starfleet Academy to make good on his promise as a star pupil. Among contemporaries like Captains Pike and Garrovick, Garth became known as an eminent explorer, with a distinguished record for charting new worlds. As a tactician, he was a pivotal figure in the Federation victory at the crucial Battle of Axanar. His exploits were immortalized in his own time, becoming required reading at the Academy and inspiring members of a new wave of young officers.
His legacy was tarnished following the accident that left him badly maimed. The gentle beings of Antos IV nursed him in his darkest hour, and gifted him with their technique of cellular metamorphosis to repair his mangled body. In gratitude, he "offered them the galaxy" – an offer refused. By this point, he was no longer the honored Captain of the history books, and his diseased mind found such offense in Antos' rejection that he ordered his forces to annihilate the entire species. Whatever loyalty he once inspired in his crews vanished, and they refused his order. Federation authorities deemed it appropriate that Garth be sent to an asylum for the criminally insane. By 2269, he was a ward of Elba II.
Exile on Elba II
The isolated asylum was a formidable cage to house the fifteen incorrigibly dangerous and unstable outcasts of Federation society. Beyond internal security measures, the facility was surrounded by a powerful force field that blocked transporter activity and could even withstand the orbital bombardment of a starship. Outside the asylum dome on the planet's surface was no option considering Elba II's poisonous atmosphere. Effectively blocked from escape, Garth was limited to whatever freedoms he could find within the asylum.
Garth's sanity may have abandoned him, but his cleverness remained. In the time following his convalescence on Antos IV, Garth had secretly expanded his control of the transformative gift that had saved his life. Far beyond repairing damaged flesh, Garth amassed enough skill to assume the appearance of others, down to the sound of their voice and their clothing. He revealed his talent by taking the form of the asylum's Governor, Donald Cory, and convincing a guard to release him from his cell. The inmates were soon running the asylum, and all of them swore allegiance to their benefactor, "Lord" Garth.
The self-styled potentate stoked his dreams of empire-building while he reinforced his control over a petty kingdom. He manufactured a chemical explosive of substantial strength from available materials (and claimed it was the most potent weapon in history), and transformed a rehabilitation chair into an instrument of torture – taking great pleasure in tormenting his former overseer, Governor Cory. He found the time to court the most beautiful woman on the planet, eventually taking her as his "royal consort". The pathologically lying green-widow Marta consistently tested Garth's infantile patience, but he endured her, as the seductive dancer was the only woman on the planet.
Queen to Queen's level three
Garth's best chance to escape his exile came as the USS Enterprise (NCC-1701) entered orbit around Elba II for the delivery of revolutionary new medicines. Garth greeted Captain James T. Kirk and his First officer Spock in the genial guise of Donald Cory, and presented the asylum's image as the visiting officers expected, including viewing inmates in their cells. All except Garth's supposed cell, inhabited by a practically crucified Governor Cory. Once the Enterprise officers were secured, Garth revealed himself and declared his ambitions for galactic domination, backed by his new cadre of followers, "the future masters of the universe".
The form of Kirk suited his long awaited moment of escape, and Garth contacted the Enterprise, asking to be beamed aboard. Chief engineer Scott, at the conn, answered the request with a chess-code phrase, "Queen to Queen's level three", dashing Garth's hope for a prompt escape. Garth raged that his great escape had been so easily countered. Garth insisted he would "shatter every bone in Captain Kirk's body" to gain control of the Enterprise. Moments later, he invited Kirk and Spock to a little dinner and light entertainment.
Kirk, who professed to hero-worship, was unmoved by the fundamental righteousness of Garth's cause. Flattery and offers of command in Garth's fleets were answered with baffling moralizing about peace and brotherhood, and reminders of Garth's former life. A subtle segue into Kirk's thoughts on chess revealed nothing, Faithless Marta's attempt to seduce (and kill) Kirk, failed. Impersonating Spock, in a third attempt to learn the chess counter-phrase, was useless. A magnificent self-coronation ceremony and bestowing the honor of "heir apparent" left Kirk un-awed. Even sacrificing Marta to the elements of Elba II, and mercifully using her to demonstrate a sample of his new explosive, bore no fruit.
Garth's final gambit, once again assuming Kirk's visage in the hopes of confusing his First officer, was unable to prevent Spock's eventual identification of his true Captain. Garth was subdued, and his followers were returned to their confinement. Cory and Dr. McCoy began to administer the new drug therapy to the asylum's patients with optimistic hope.
Before the Enterprise departed, Kirk was at last able to catch a glimpse of the hero of his youth. With the help of the new medicine, Garth seemed to be emerging out of a fog, somewhat bewildered. Kirk identified himself and the name meant nothing, but the words "starship" and "captain" sunk in, and clearly held meaning. Garth raised his arm to grip the hand of a starship captain. (TOS: "Whom Gods Destroy")
Garth's mirror universe counterpart appears in the novel The Sorrows of Empire; he is the Grand Admiral of the Imperial Starfleet and, like his prime universe counterpart, he possesses the shape-changing ability taught by the Antosians. Correctly guessing the true nature of Spock's rise to power using the Tantalus field, Garth runs afoul of the same chess solution that foiled his prime counterpart in "Whom Gods Destroy", before he is himself "disappeared" by the device - courtesy of Marlena Moreau. His co-conspirators involved the mirror counterparts of Marta and Governor Cory. Garth is succeeded as commander of Starfleet by the mirror counterpart of Matt Decker.