The Aschen War was a series of brushfire revolts and Chaos incursions that erupted across the Aschen Sub-sector between 300.M40 and 307.M40, after a bloody Chaos Cult-inspired revolt on the Cardinal World of Dimmamar exposed how overstretched Imperial control over the surrounding region had become. Slowly stripped of much of its defences over the course of several generations to provide for the needs of other pressing conflicts nearby, this border region was ill-equipped to stave off catastrophe when it came. Very rapidly a three-way war developed between the beleaguered Imperial protectors of the Aschen worlds, Chaos Cultists who had subsumed several planetary defence garrisons to their cause and Ork marauders pouring across the border from the adjacent Blood-Maw Cluster.
The Imperial counter-invasion was led by the combined forces of two Space Marine Chapters -- the Exorcists and the Dark Hands, backed by the Necromundan 54th and 60th Imperial Guard Rifles and the Battle-Sisters of the Order of the Valorous Heart. The Exorcists quickly realised that one of the keys to victory would be locating and destroying the single dark master of many names that lurked behind the divergent Chaos Cults in the region, a daemonic creature that on Dimmamar was known as the Horned God. A thing of shadow and malice, the Horned God was a master of hellish pacts who had become engorged on the souls its servants had slaughtered. When the Exorcists finally cornered the Daemon Prince in the maze-like pre-human ruins of the Dead World of Belphago, it was attended by a guard of thousands of its fanatical followers and several warbands of Night Lords Chaos Space Marines it had bound to its will.
What followed was one of the hardest fought and costly actions in the Chapter's history, as the Exorcists, with the aid of a hastily composed Ordo Malleus strike team, fought their way through suicidal attack waves of cultists and the skilful ambushes and vicious assaults of the Night Lords before confronting the blackly-burning daemons that served the Horned God. In the final, titanic struggle amid the fallen ruins of a xenos temple to the Dark Gods older than the history of Man, the Exorcists' elite Enochian Guard and a handful of surviving Inquisitors and their Acolytes confronted the Lord of Shadows in all its blasphemous and unholy glory. Shrugging off shot, shell and energy blast, the darkling beast slashed through the Exorcists' ranks, shearing Terminator-clad warriors in half with contemptuous ease and ripping asunder the Daemonhosts of Inquisitor Grey before devouring their master whole.
The Exorcists refused to retreat from the horror that confronted them despite their losses, and slowly the weight of their attacks began to take their toll. The turning point of the battle came when one of the Chapter's Dreadnoughts, the Venerable Sybra, sacrificed himself to rip a great chunk of the Daemon Prince's smouldering body away and exposed its blazing heart. This noble act allowed the Exorcists to press their attack while the daemon reeled in agony. With a daring lunge, Librarian Malachite pierced the foul heart with his Force Spear, and with the utmost exertion of his powers and adamant will through the weapon severed the daemon's connection with the mortal plane, finally vanquishing it.
As the Daemon Prince's cries of rage and frustration echoed into eternity, all across the Aschen Sub-sector, those who had bartered their souls for a measure of the Horned God's power paid the price and were consumed by black flames, shattering the power of the Chaos revolt. Fully a third of the Exorcists Chapter had perished, but the tide of the war was now in the Imperium's favour, and within a year, the sub-sector was brought back under Imperial control.
- Imperial Armour Volume Ten - The Badab War - Part Two, pg. 126