The Quelii Oversector was the theaters of operations of the Republic 10th Sector Army during the Clone Wars. After the Declaration of a New Order, it was reorganized in an Imperial Priority Sector and placed under the rule of Grand Moff Tanniel.
The Oversector had presumably been created to prosecute the ongoing war against the warlike Drackmarians, who were the dominant species of the Sector, but it also contained Dathomir, with its Force-strong population, strategic mineral resources, and enigmatic lodes of lost knowledge and technology.
The Quelii Oversector was later given to Zsinj, Moff of the Quelii sector, in recognition of his brilliant career. In this new position, one of Grand Moff Zsinj's duties was to conquer the Drackmarians and control the Rimward stretch of the Hydian Way. To this end, his forces was supplemented with Crimson Command's refitted Victory-class Star Destroyers along with the Executor-class Star Dreadnought, Brawl, making his Sector Group the largest fleet in the Empire.
After the Battle of Endor, when the Imperial Ruling Council recalled the Quelii Oversector's Sector Fleet to the Core, Zsinj ignored the order. Instead, he declared himself Warlord and adopted the white uniform of a Grand Admiral, embarking on a war of conquest that brought a third of the former Empire under his control, in a sprawling tyranny with the Quelii Oversector at the heart of his empire.
Subsequently, it was in Quelii space that the New Republic won two major defeats against Zsinj, at the Battle of Selaggis and the Battle of Dathomir, and subsequent New Republic influence was responsible for reforms such as freeing the Chev from slavery to the Chevin on Vinsoth. Even before Zsinj's defeat, however, the Empire had made inroads to the area as far as Halmad, subsequently reoccupying Dathomir in 12 ABY and remaining a threat in the region until at least 23 ABY.