From: Star Trek: The Original Series
Planet: Triskelion. Members of a species who apparently were humanoid in appearance, but eventually evolved into disembodied brains due to their devotion to intellectual abilities. Three such disembodied brains dwelled below the surface of the planet Triskelion, and control the events which transpire on the surface of the planet for the purposes of their own entertainment — which is namely to capture humanoid species (termed "thralls") for combative games so that the Provider's can wager on the winners. On stardate 3211.7, Captain James Kirk and two of his crew were captured in this manner.