The USS Defiant was a starship from the mirror universe that was in service in the 23rd centery. It encountered an interphasic rift that sent the ship into the prime universe where it was first captured by the Tholians, who detonated a tri-cobalt warhead in the gravity well of a dead star to create the interphase rift, then sent a distress call through to see who would respond. It was then stolen from them by the Terran Empire, and was later used by the Empire to crush the Anti-Terran Rebellion.
Mirror universe origins
Built at Tranquility Base, Luna, the Defiant was a Constitution-class heavy cruiser that was in Federation service during the mid-23rd century. At some point, the Defiant encountered an interphasic rift.
The Defiant emerged in this universe during the 22nd century, where it was discovered that Tholians in that universe had created the interphasic rift by detonating a tri-cobalt warhead within the gravity wel of a dead star, and had sent the distress call which had lured the Defiant to the area. The Tholians, soon after, took the ship to a drydock built into an asteroid-moon in orbit around a gas giant in the Vintaak system and began a salvage operation, but were unable to finish because the ship was stolen by Commander Jonathan Archer of the ISS Enterprise. (ENT: "In a Mirror, Darkly")
After Enterprise was destroyed in a Tholian web, Archer (now aboard the Defiant) made a desperate attempt to escape by restoring shields and weapons. After the station tractored the Defiant, Tucker was able to restore those systems. The Defiant began firing at the Tholian station and disabled it. At first, the Defiant could only reach speeds at impulse because a Gorn stole one of the key components from the engines (a plasma regulator). The Gorn was killed by Archer, who was able to restore warp drive.
Instrument of the Ambitious
After arriving at the rendezvous point with the assault fleet, Archer found two Andorian battle cruisers, one Vulcan D'Kyr-type cruiser, and one Tellarite cruiser were disabling the ISS Avenger. The Defiant destroyed all rebel ships except one Andorian cruiser which Archer intentionally allowed to escape.
En route to Earth to present the Defiant to the Emperor, the freedom fighters Soval and T'Pol tried to sabotage Defiant with Phlox's help. This act was part of the Anti-Terran Rebellions final attempt to defeat the Terran Empire and was inspired by the discovery that in the Federation of the mirror universe, Vulcans and Denobulans were equal to Humans. When Phlox disabled main power, the Defiant was defenseless, and the Avenger, commandeered by Soval and other rebel species, began firing at her. Defiant received heavy damage until Tucker was able to restore main power. With power restored, Archer ordered the destruction of Avenger. The Defiant fired phasers and photon torpedoes and destroyed Avenger's warp regulator. The second barrage of photon torpedoes was targeted at their main reactor and the Avenger was destroyed with all hands.
After destroying Avenger, Defiant arrived at Earth, now under the command of the self-proclaimed "Empress Sato", having poisoned Archer and taken command of the ship. She ordered Admiral Gardner to surrender and stand by for instructions, or she would open fire on Earth. (ENT: "In a Mirror, Darkly")