Located in Ultima Segmentum and at the convergence point of several Warp routes (collectively known as the Voidspan Passage), the Corvus Sub-sector is a strategically important region and has long been an influential player in interstellar trade. It was also a major base for Battlefleet Ultima, which anchored off Voidspan Passage and controlled the region.
Disaster eventually befell the Corvus Sub-sector when a giant Warp Storm known as the Crow's Eye enveloped the entire region as well as a number of outlaying Systems. The whole Subsector was soon cut off from the wider Imperium for almost exactly a millennium. Denied the succor and trade of the Imperium as well as the military and logistical support of the Adeptus Terra, starvation and anarchy erupted throughout the Subsector. Worse still, the inhabitants of the Warp, namely Daemons and Chaos Space Marines, arrived to wreak havoc alongside raiding parties of Dark Eldar which emerged from the Webway. Unknown to the besieged denizens of Corvus however was that this course of events was caused by the sinister Cult of Amber.
After a thousand years of this hell, the Crow's Eye suddenly vanished as quickly as it had arrived. The Imperium launched an expedition to restore order to the region, dubbed the Crusade of Fire, coming into heavy fighting with Chaos and Dark Eldar forces. The Crow's Eye did not recede all at once, so at first only the outermost planets of the sub-sector were reachable.