Galt was the master thrall on Triskelion in 2268. He was in charge of humanoids that were brought to the planet to fight as gladiators in combat games in order to entertain and provide gambling for the disembodied entities called the Providers.
His position let him control the levels of pain the drill thralls felt from their collars. He controlled this without any gesture, only his eyes glowed as the pain was inflicted. He also had a limited means of individual teleportation and he could appear at any place to summon and direct the thralls. He used these means to pacify and control the thralls, such as stopping an escape attempt by James T. Kirk and his landing party.
After Kirk had won a wager from the Providers by defeating three thralls that won the landing party their freedom, Galt was left on the planet with the other former thralls to begin a new life of freedom. (TOS: "The Gamesters of Triskelion")