A planet in the trinary star system M24 Alpha where three disembodied brains, the Providers, wager on gladiatorial fights between captured thralls.
Triskelion is a world ruled by the Providers, three ancient beings who have become living brains hidden beneath its surface. With little else to occupy them, they abduct beings from across space to fight as drill thralls and bet on the results in a currency called quatloos.
In The Gamesters of Triskelion, Kirk, Uhura and Chekov are transported there from the Enterprise and trained as thralls. Kirk eventually wagers that he can defeat three opponents. When he wins, the Providers agree to free the thralls and help them build their own society.