After the Battle of Endor, with the death of Emperor Palpatine, and Darth Vader, the Galactic Empire went into chaos. Moffs, Admirals, and Generals became warlords and carved their own territory and made their splinter kingdoms. The Imperial war machine, was disabled with all of the warlords seizing whatever resources they could get their hands on.
In 8 ABY, the return of Grand Admiral Thrawn, one of the last of the Grand Admirals, returned from his adventure from the Unknown Regions maintaining Empire of the Hand. Thrawn was able to gather up a large some fleet that could threaten the New Republic. With the New Republic’s capture of Coruscant, Thrawn would be elected Supreme Commander of the Imperial Fleet by the Imperial Ruling Council on Orinda.
The fleet would participate in many battles. Notably when Thrawn attacked Sluis Van shipyards, and up until the Battle of Bilbringi with the death of Thrawn. The remains of the fleet would flee to the Unknown Regions, when Thrawn’s protégé, Captain Pellaeon would take command.
During Operation Shadow Hand, with the resurrection of Palpatine, the fleet, still underneath the command of Pellaeon would be ordered to capture the planet Duro, there, much of the fleet would be destroyed.