- Warning: The following content contains spoilers!
In the mirror universe, the agony booth was a device used by the Imperial Starfleet of the Terran Empire to discipline members of the fleet for serious infractions. For the more relatively minor infractions, an agonizer was used instead. The mirror universe version of Spock called the booth a most effective means of discipline. (TOS episode: "Mirror, Mirror")
The agony booth was first invented in the mid 22nd century by the MACO Major Malcolm Reed and Doctor Phlox on board the ISS Enterprise. Reed and Phlox first tested the device by torturing a Tellarite engineer in the device. Captain Maximilian Forrest was at first skeptical, feeling that the traditional flogging would be a better form of discipline until Phlox explained that while a person would eventually become desensitized to flogging, the agony booth was designed so that victims could never become accustomed to the pain. A number of other people were subjected to the device - including Charles Tucker III, Jonathan Archer, and even Malcolm Reed. (ENT episode: "In a Mirror, Darkly")
In the 23rd century, agony booths were in use throughout the fleet. The ISS Shenzhou had three agony booths in the ship's brig. While posing as his prime universe counterpart, Gabriel Lorca was tortured inside one of the Shenzhou's booths. Several of Lorca's followers such as Commander Ellen Landry were held in agony booths aboard the Terran flagship, ISS Charon. (DSC episodes: "Despite Yourself", "What's Past Is Prologue")
The ISS Enterprise also had an agony booth. After a failed attempt to assassinate Captain James T. Kirk of the primary universe, Pavel Chekov was subjected to the agony booth. Kirk came across Chekov being tortured in the agony booth. He ordered his personal guards to have Chekov released and confined to quarters. (TOS episode: "Mirror, Mirror")
In one version of mirror reality, the mirror Kirk had Chekov placed back in the booth. He had the power level of the booth turned down, but left Chekov in the booth until he died thirteen days later. After Chekov died, his dessicated corpse was photographed and the photo was framed. The macabre photo was hung in Tiberius' hideout in Iowa by 2375. (TOS novel: Dark Victory)
In another version of the mirror universe, the ISS Enterprise-D had an agony booth as well. Security chief Deanna Troi used the device to torture Geordi La Forge when she discovered that La Forge was behaving suspiciously. (TNG novel: Dark Mirror)