Garth of Izar - Pamela Sargent

Captain Garth of Izar, hero of the battle of Axanar which helped secure the future of the Federation, was once a legendary Starfleet Captain whose exploits were required reading at Starfleet Academy. In the 2260s he suffered serious injuries in an incident on Antos IV. The inhabitants of Antos repaired his body by teaching him the art of cellular metamorphosis, but did not realise that his injuries had rendered him criminally insane.
After ordering the destruction of Antos IV, Garth was committed to the Federation rehab colony on Elba II, where, in 2268, he overpowered his keepers, escaping with the aid of the cellular-metamorphosis process, mastery of which enabled him to take on the appearance of any person he wished. Before his eventual recapture he had proclaimed himself lord of the universe and attempted to commandeer the Starship Enterprise.
Now cured of his insanity by new techniques of experimental medicine, Garth of Izar must rebuild his shattered life. The opportunity to do so is provided on a mission to solve a diplomatic crisis -- a crisis that requires Garth to return to the very planet that caused his insanity so many years ago.