The agonizer is a small device carried by crews in the Mirror Universe's Terran Empire and used to inflict punishing pain on subordinates.
Discipline aboard the ships of the Terran Empire depended on fear. Each crew member carried an agonizer, which superiors used to punish errors by applying intense pain. For more serious offenses the Empire used the agony booth, a chamber that inflicted sustained torment.
Captain Kirk and members of his landing party discovered these practices in 2267 when an ion storm exchanged them with their counterparts and placed them aboard the brutal ISS Enterprise. The devices became emblems of the Mirror Universe in later stories.