From Destinypedia, the Destiny wiki
Prisoner Exchange is a World Event in Destiny 2 that is only available during Flashpoints. On worlds with a Vex presence, such as Nessus, Io and Mercury, a Minotaur Guard guarding an energy cage with an Ultra will appear. Eliminating the Minotaur Guard will drop nothing, but will make the energy shield around the Ultra disperse, releasing it. Defeating this enemy will drop a loot chest which could contain reputation tokens, engrams or pure loot. In Flashpoints, sometimes the health and loot drops of the Ultra will increase, or more enemies will be added to the cage. For example in the Fields of Glass Prisoner Exchange on Mercury, three Psion Flayers will join the Brawl Gladiator in the cage.
List of appearances