In the mirror universe, an agonizer was a device developed and used by the Terran Empire's Imperial Starfleet to punish their members for relatively minor offenses. It was invented by Dr. Sarah April in the 23rd century, and its first human test subject was Captain Nathan Thorpe. Its initial tests against Klingons proved to be a failure. (TOS short story: "Ill Winds")
Agonizers were worn on a person's belt, and could be demanded by any superior officer at any time if they felt a subordinate had committed an infraction. When the agonizer was applied to the body it would cause intense pain which could bring a person to their knees. (TOS episode: "Mirror, Mirror")
In an alternate version of the mirror universe, agonizers were still used by the Terran Empire in the 24th century, and had been refined to the point that they were incorporated into the Starfleet insignia worn on a person's uniform. As Deanna Troi was head of ISS Enterprise-D security, a special agonizer was designed specifically to function with Geordi La Forge's VISOR. (TNG novel: Dark Mirror)
In 2273, an improved model was placed near the head and generated wave pressure to the fifth cranial nerve of the brain. First officer Koth of the IKS Kandar threatened James T. Kirk with a demonstration of this new agonizer if the captain did not hand over reserve dilithium crystals from the USS Enterprise. (TOS comic: "Dilithium Dilemma")
- TOS episode: "Mirror, Mirror"
- TOS episode: "Day of the Dove"
- DS9 episode: "Shattered Mirror"
- TOS comic: "Dilithium Dilemma"