Sotek was a High Master of Kolinahr during the Vulcan Reformation. Born as Khoteth, he was trained in mind-powers by the High Master Zakal, a feared and powerful mind-lord, becoming like a son to him. However, Khoteth turned against his master and followed Surak's teachings of peace and logic, adopting the name Sotek. He imprisoned his former master, and his initiates curbed Zakal's immense powers. As Zakal neared death from lunglock fever, Sotek came to honor a promise he had made to his old mentor, to preserve his katra in the Hall of Ancient Thought, knowing that Zakal was too vain to allow himself to taste true death.
Around two thousand years later, in the Earth year 2271, Sotek's katra was carried within the body of the High Master T'Sai, who had gone with Spock and Dr. Keridwen Llewellyn in pursuit of Sekar, a renegade Kolinahru who carried Zakal's katra within himself and planned to bring the mind-rules to the Romulans. Encountering Zakal's commandeered Romulan Bird-of-Prey, T'Sai - injured by a telepathic attack from Zakal - transferred Sotek's katra to Dr. Llewellyn, giving her the ancient mind's immense powers and, because of her esper-blindness, allowing her to transport onto the ship without Zakal's knowledge. Setting the Bird-of-Prey to self-destruct and holding Zakal's attention long enough to prevent him from stopping it, Sotek and Dr. Llewellyn sacrificed themselves willingly to stop the evil mind-lord. Though Sotek's katra was lost, his sacrifice ensured that the mind-rules would not get into the hands of the Romulans and be used against Vulcan. (TOS - The Lost Years novel: The Lost Years)