The ISS Endeavour is a 24th century Terran Empire Galaxy-class starship operated by Starfleet. It was originally known as the USS Axanar, and served the United Federation of Planets when an ion storm brought it to the mirror universe.
The Axanar was patrolling the Beta Quadrant border with the Alpha Quadrant when an ion storm struck, sending it into another universe and several hundred lightyears away. Not realizing the transfer, the ship sent out a distress signal, which the ISS Icarus responded to.
The captain, Alex Garth, attempted to request medical assistance, but was denied. Instead, the crew of the Icarus boarded the Axanar and slaughtered the entire crew, including Garth. Gemini Shadow took command from then on.
The Axanar was later renamed the Endeavour and assigned the registry number of NCC-0326, and spent several months in space dock undergoing renovations, such as adding Terran Empire insignias to the exterior hull and various interior surfaces, such as carpets, doors, and walls.
After renovations, the Endeavour headed into space for its first proper mission, negotiations with the Halkans over mining rights. During this time, the away team was transferred to the smaller, Miranda-class USS Endeavour due to a transporter malfunction, though they quickly returned. (NE 1.05: "The Dark Dimension")
Raven Shadow later took command of the ship in a mutiny against her older sister, and remains the captain to this day.