Thann was a Vian in the 23rd century and was among the Vians who, in 2268, tested Gem to determine if her species was worthy to be saved from the impending nova of their star Minara. He possessed the ability to transport the entire population of Gem's species to a safe location.
He possessed the ability to read the minds of humanoids. Thann possessed hand-held devices capable of inflicting torture, healing powers and transportation that were attuned to the thought waves of the individual carrying them. He was all intelligence, no emotions, and ruthless. He would sacrifice any number of humanoids to see if Gem was worthy to save.
He and his companion kidnapped James T. Kirk, Spock, and McCoy in order to test Gem's compassion and self-sacrifice. They were tortured and McCoy was almost killed. When he was convinced that she had the quality that would establish a quality civilization, he healed McCoy and saved her world. (TOS: "The Empath")