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The '''{{PAGENAME}}''' was a major battle fought by the [[Imperium]]. Some time after the [[Thirteenth Black Crusade]].{{Fn|1}}

The invasion began after the creation of the [[Great Rift]], during the [[Thirteenth Black Crusade]], when the [[Chaos God]] [[Tzeentch]] grew jealous of his its Brother [[Nurgle]]'s newly acquired domain of the [[Scourge Stars]]. While Tzeentch began to underman Nurgle's control of the Scourge Stars, the Chaos God also wished to corrupt a realm of space for itself and selected the [[Imperium]]'s [[Stygius Sector]] to be its domain. This was due to the fact that Stygius Sector was still awash with [[Warp Storm]]s created by the Great Rift's openingand as a result, and the light of the [[Astronomicon]] could no longer be seen there; all of which, made the Sector an inviting target for the Chaos God's forces.{{Fn|1}}

The opening phase of the invasion saw [[Cult]] uprisings begin across a score of the the Sector's isolated worlds and the actions of the [[Changeling]] added to the Sector's troubles, as the [[Daemon]] traveled from world to world , spreading havoc and confusion. However, it was only when the Stygius Sector's worlds were seething with rebellion that the true invasion began, as Tzeentch's [[Daemon]]s and [[Chaos Space Marine]]s were unleashed upon the Sector. With the [[Daemon Primarch]] [[Magnus]] and his [[Thousand Sons]], along with the [[Lord of Change]] [[M'Kachen]] and its [[Great Changehost]], among Tzeentch's forces, three of the Stygius Sector's [[System]]s were swiftly taken, ; with only the Imperium world [[Mordian]] being able to resist the Chaos God's invasion. Shortly afterwards though, the light of the Astronomicon, began to miraculously reach the Sector and its beleaguered defenders immediately sent out a request for aid.{{Fn|1}}

When the Imperium received the request for aid, the [[Stygius Crusade]] was hastily assembled and sent to aid the Sector, but when it arrived seven sub-sectors had already fallen in into Tzeentch's into hands. Despite being heavily outnumbered, the Stygius Crusade immediately fought to stem any further losses to the Chaos GodTzeentch's invasion and launched attacks to reclaim the worlds already claimed by Tzeentchthe Chaos God's forces. The battles that followed were epic in size and savagery, but though the Crusade was able to break Tzeentch's siege on Mordian, they were hit with several devastating setbacks. Chief among them, was that due to the Great Rift and the Sector's Warp Storms, several of the the Crusade's fleets were lost, when they were seized by the [[Warp]] and hurled into the distant past or future. The Warp also claimed the Imperium world [[Dakidda]] and with it, the entire [[44th Necromundan]] [[Regiment]] which had just combat dropped onto the invaded world. This was just the start of the Crusade's downfall, as Tzeentch's forces began to overwhelm them the Crusade's forces and even the dead rose up to aid the Chaos God 's conquest of the Stygius Sector.{{Fn|1}}

However, just as the Crusade began to fall to Tzeentch's might, a large strike force from the [[Craftworld]] [[Ulthwe]] arrived in the Sector without warning and began to push back the Chaos God's forces. The [[Eldar]] did not arrive to aid the Crusade against the forces of [[Chaos]] though. Instead, the Craftworld's [[Seer Council]] contacted the Crusade's commanders and told them they could not save the Stygius Sector and staying there would not aid the Imperium. Indeed, the Seer Council claimed the visions they had received, showed only the Crusade's destruction if they stayed and warned that Magnus knew this as well. Despite the mistrust the Imperium had for the Eldar, the Crusade's commanders agreed to retreat and evacuated what forces they could, as the Eldar's strike force acted as a rearguard against Tzeentch's hordes. Eventually though, the Crusade and Eldar were both forced to retreat and the Stygius Sector became the first of the domains, Tzeentch planned on claiming for itself.{{Fn|1}}