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.
After the Battle of Endor, the military resources deployed to the Quelii Oversector, led by the Executor-class Star Dreadnought Brawl, constituted the most powerful single combat force in the Imperial Navy. 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.
After Endor, when the Imperial Ruling Council recalled the Quelii Oversector's Sector Fleet to the Core, its commander, Admiral 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 its heart.
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.
During the Yuuzhan Vong War, parts of the former Oversector were overrun early in the onslaught, but resistance continued throughout the war.