One of the two Vians who test the empath Gem and torture Kirk, Spock and McCoy to measure her compassion.
Thann is one of the Vians, a powerful alien people with large heads and advanced technology. In The Empath, he and his colleague Lal capture Kirk, Spock and McCoy in their laboratory beneath the surface of Minara II, where they have already killed two Federation researchers.
The Vians intend to save one of the inhabited planets in the system, which is threatened by its star's nova, and are testing the empath Gem to learn whether her people deserve it. Kirk finally convinces Thann and Lal that their own lack of compassion makes them unfit judges, and they agree to save Gem's people. Thann was played by Willard Sage.