The Byss Defense Fleet was dedicated to the defense of the planet Byss, Emperor Palpatine's secret retreat. Despite this limited area of operations, the fleet was large enough to protect an entire sector. Moreover, the fleet included some of the largest ships in the Imperial Fleet. It was made of various types of starships, including TIE/LN starfighters and GAT-12 Skipray Blastboats, as well as capital ships such as several Imperial-class Star Destroyers, at least one Secutor-class Star Destroyer, at least one Procursator-class Star Destroyer, at least two Vengeance-class dreadnoughts and at least one Bellator-class dreadnought. The elite TIE Fighter unit Dark Squadron was also attached to the fleet.
During the early years of the Galactic Empire, the fleet helped to guard the Super Star Destroyer prototype, which was under construction above Byss. The fleet fought against an Alderaanian Resistance fleet during the Battle of Byss, when the Resistance attempted to destroy the prototype.
Decades later, the fleet was still stationed around the planet during Operation Shadow Hand, around 10 ABY. The fleet was eventually annihilated during the destruction of Byss in 11 ABY.