The Terran Empire NX-class starship and flagship of Admiral Black in the mirror universe, rescued from rebels by the captured USS Defiant in 2155 and then destroyed by it during Archer's bid for power.
The ISS Avenger is a Terran Empire starship of the NX class seen in In a Mirror, Darkly, Part II, the second half of the mirror universe story in Star Trek: Enterprise. She is the flagship of Admiral Black, and the mirror Soval serves aboard as a science officer.
In 2155 the Avenger is losing a battle against rebel ships when the mirror Jonathan Archer arrives in the USS Defiant, a 23rd-century ship from the prime universe, and destroys the attackers. The survivors of the ISS Enterprise are taken aboard the two ships. Archer then kills Black and announces to the Avenger's crew that he means to take the throne.
T'Pol and Soval lead the non-Terran crew in a plot to cripple the Defiant, with the Avenger attacking her. The plot fails and the Defiant destroys the Avenger.