The fleet of the Terran Empire in the mirror universe, a force of conquest whose officers advance by assassination and whose ships carry the prefix ISS.
Imperial Starfleet is the dark counterpart of the Federation's service. In Mirror, Mirror, Kirk and three of his officers find themselves aboard the ISS Enterprise, where discipline is enforced with agonisers, captains keep personal guards and killing a superior is an accepted route to promotion.
Enterprise shows its 22nd-century form in In a Mirror, Darkly, in which Commander Archer seizes the captured USS Defiant and Hoshi Sato ends by declaring herself Empress.
Discovery explores the fleet of the 2250s, including the ISS Shenzhou and the palace ship ISS Charon of Emperor Philippa Georgiou. By the time of Deep Space Nine's Crossover the Empire has fallen and its fleet is gone.