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 Tyranid hive fleet Leviathan, suffered several subsequent catastrophes. Subsector Aurelia, a strategically important Imperial bastion along the Fringe, and home of the Blood Ravens chapter of Adeptus Astartes, faced the brunt of these uprisings, in what are now known as the Aurelian Crusades.
In 001.M42, a Black Legion warband led by Araghast the Pillager, sought to unleash the daemon, Ulkair, upon Aurelia's inhabited worlds. This act would trigger the Second Aurelian Crusade, which lasted several bloody months and threatened to wrest control of Aurelia from the Imperium. At great cost, Araghast's host was defeated, and their numbers scattered across the subsector. However, this victory was short lived, as new troubles arose to plague Aurelia in the aftermath of the Black Legion's attack.
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, capital of the southern hemisphere. With the Blood Ravens deployed to various warzones along the Eastern Fringe, Meridian's defense 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, the assault had stalled, resulting in heavy casualties among the Vendoland regiments. The blow to Imperial pride would result in a renewed wave of violence across the subsector. Imperial Guard forces were deployed en masse to Aurelia to pacify the troubled region, marking the beginning of ten long years of struggle.
This period of time is recognized as the beginning of the Third Aurelian Crusade. The pacification of Meridian would be a long struggle that came to signify the early years of the Crusade. It is also during this time that many notable Imperial Guard regiments faced their first trial by fire.
Excerpt from "The Early Crusades of M42" (Inquisitor M. Adrastia)