A metal collar locked around the neck of every thrall on the planet Triskelion, able to inflict agonising pain or death at the will of the master thrall Galt.
Collars of obedience appear in the episode The Gamesters of Triskelion. Kirk, Uhura and Chekov are snatched from the Enterprise and fitted with them on arrival, joining captives from many worlds who are trained to fight for the amusement of the Providers.
The collars cannot be removed by the wearer. Galt, the master thrall, activates them simply by concentrating, his eyes glowing as the victims collapse. Once a thrall is bought by a Provider the collar shows that owner's colour.
Kirk wagers the freedom of his crew and all the thralls on a fight against three opponents. When he wins, the Providers, who turn out to be three disembodied brains, keep their word, agreeing to free the thralls and teach them to govern themselves.