In the early years of the 42nd Millennium, war continued to rage across the Eastern Fringe of Segmentum Ultima. The Korianis Sector, still recovering from the ravages of a splinter tendril of the dreaded Hive Fleet Leviathan, suffered multiple subsequent calamities, known now as the Aurelian Crusades. The target of this aggression was Subsector Aurelia, a strategically important Imperial holding along the Fringe, and the vital recruiting worlds of the Blood Ravens, a chapter of the Adeptus Astartes.
In 001.M42, a Black Legion warband led by Araghast the Pillager assaulted Aurelia's planetary systems, seeking to unleash the bound Daemon, Ulkair. This act would trigger the Second Aurelian Crusade, which would last several bloody months and threaten to wrest control of the subsector from the Imperium's grasp. Through great cost to both the Blood Ravens and the assisting Imperial forces, the Araghast's host was defeated, and their numbers scattered across the subsector. However, this would not be the end of troubles to plague Aurelia.
On the sector capital of Meridian, noble houses that had sided with the traitor legions rose up against the unpopular Imperial government, committing their Houses' considerable military power to the cults. The most powerful of these, the Hounds of Vandis, sought to challenge the Imperial forces in open combat. The personal army of the former governor, Gregor Vandis, the Hounds took control of Hive Urizen, Meridian's stronghold in the southern hemisphere. With the Blood Ravens deployed to various warzones along the Eastern Fringe, the duty fell to the local Imperial Guard regiments to take the fight to the Hounds. Three regiments of the Vendoland Guard; the 36th Recon, 46th Armored and 85th Infantry, moved on Urizen, intent on driving the traitors from the city.
This proved to be disastrous. Within a week of fighting for the outskirts of the Hive, the Vendoland regiments became bogged down in deadly street to street fighting, resulting in massive losses. This blow would set off a series of renewed battles for the war torn planet. Imperial Guard forces were deployed en masse to Subsector Aurelia to pacify the troubled region of space, but it would not be until 003.M42 that the conflict would subside.
This period of time is recognized as the beginning of the Third Aurelian Crusade. These early years were marked by numerous engagements across Meridian. The various factions vying for control would soon settle into established zones of control that would remain unchanged for nearly ten years. It is also here that many notable Imperial Guard regiments faced their first trial by fire.
Excerpt from "The Early Crusades of M42" (Inquisitor M. Adrastia)