An advanced species that forced Kirk and the Gorn captain to fight.
The Metrons are a highly advanced species whose true appearance is not shown on screen, although one Metron appears to Kirk in humanoid form, and their true nature is never explained. They have enormous technological powers, which allow them to transport entire starships across space and to monitor activity from a great distance.
Their society is quiet and aloof, and they regard violence as something to be controlled rather than celebrated. They take a keen interest in how other species behave, and they have the power to place unwilling spectators in a situation where they must resolve a conflict.
They appear in the original series episode Arena, in which the Metrons end a fight between the Enterprise and a Gorn vessel by taking both captains to a nearby planet and making them fight to the death. Captain Kirk uses ingenuity to improvise a weapon and then chooses mercy, which impresses the Metrons, who say humanity may one day be ready to meet them.