Garth of Izar was a legendary Starfleet captain who went insane, gained shape-shifting powers and tried to take over the Enterprise from an asylum on Elba II.
Garth of Izar was once one of Starfleet's greatest captains, and his victory at Axanar was required study at Starfleet Academy, where James T. Kirk admired him. After a serious injury, Garth was healed by the people of Antos IV, who also taught him the art of changing his shape. He later went mad and tried to destroy the Antosians, and he was sent to the asylum for the criminally insane on Elba II.
In the original series, Garth took over the asylum and used his powers to pose as the governor and then as Kirk, trying to trick his way aboard the Enterprise. Aided by an Orion inmate named Marta, he hoped to lead a war of conquest. Spock unmasked him when two Kirks stood before him, and Garth was given a new drug that began to restore his mind.
Garth is a warning about how greatness and power can turn into madness, and he showed that even Starfleet heroes could fall.