Three disembodied brains on Triskelion who abduct beings from across space to fight as gladiators, betting on the outcomes.
The Providers are an ancient species who have evolved into three bodiless brains kept beneath the surface of the planet Triskelion. They have no physical needs left and amuse themselves by collecting beings from many worlds as slaves called thralls, training them to fight and wagering on the results using a currency called quatloos.
In the episode The Gamesters of Triskelion, they abducted James T. Kirk, Nyota Uhura and Pavel Chekov and placed them under the command of a master thrall named Galt. Kirk drew the Providers into a wager in which he fought several thralls at once, and when he won they agreed to free the thralls and teach them to govern themselves.