In the mirror universe, the Magistrate was the designation of the Talosian leader who captured Captain Christopher Pike of the ISS Enterprise in the mid 23rd century. The Talosians had used a distress call to lure the Enterprise to Talos IV with the intention of having Pike mate with Vina so as to create a race of Terran slaves. However, Pike rejected her, refusing to mate with an "insipid human female."
Upon learning of the Terran Empire's fear of telepaths, the Magistrate decided to release Captain Pike and use him as their eyes and ears throughout the empire. The captain would give the Talosians an early warning should the empire ever decide to attack and obliterate their planet.
Although classified by the Imperial Starfleet, Lieutenant Commander James T. Kirk was able to learn the details about what had occurred on Talos IV through Spock in 2264. After he assassinated Pike, Kirk assumed the captaincy of the Enterprise and ordered the ship to Talos IV while he attempted to shield his mind from the Talosians by focusing on thoughts of intense rage. Upon arriving at the planet, Kirk ordered that its surface be levelled, annihilating the Magistrate and the entire race of telepaths for the good of the empire. (TOS - Mirror Universe short story: "The Greater Good")