ISS Styx

In the mirror universe, ISS Styx was a Terran Empire starship, the  dreadnought cruiser prototype in Imperial Starfleet service in the 2250s decade. When temporally displaced, the Styx was captured by the Alpha Quadrant Alliance and remained in service in the 2410s decade. 

Service history
The Styx was active by the year 2256. It was built by ingenious engineers, based on the flagship of the Terran Empire, Emperor $𝜇$'s but at a fraction of the size. It remained massive enough to store starships in its hangar bays. The frigate  was part of the dreadnought's complement of embarked craft. 

The Styx and other Terran Empire dreadnoughts of its class participated in a Terran invasion of Pahvo, travelling from the mirror universe in 2256 to the prime universe in 2410. The fleet was led by the Styx and its sister ships, and ultimately halted by defense satellites and a squadron of Alpha Quadrant Alliance ships. 

When Starfleet salvaged the ISS Styx and refitted the dreadnought cruiser to tier 6 standards in 2410, the Lobi Crystal Consortium began offering mirror carriers to Starfleet captains. Starfleet also returned the ISS Shran, found in the Styx's hangar bay, to service. 

The ship could then be deployed by AQA admirals for missions. (|Welcome to Earth Spacedock}}, admiralty system)