After a bloody cult revolt on the Cardinal World of Dimmamar in 300.M40 exposed how overstretched Imperial forces in the region had become, a series of subsequent revolts and incursions erupted across the entire Sub-Sector, eventually becoming known as the Aschen War. The border regions, gradually stripped of their military resources for use in conflicts elsewhere, were ill-equipped to deal with the multitude of uprisings and xenos invasions. Eventually, a three-way war erupted between the Imperial defenders, Chaos-worshiping cults, and Ork marauders.
After years of bloody conflict, the Imperium finally mustered a major counter-attack consisting of forces from the Exorcists and Dark Hands Space Marines backed by the Necromundan 54th and 60th Imperial Guard Regiments and the Order of the Valorous Heart of the Sisters of Battle. The Exorcists quickly realized that one of the keys to victory would be finding and destroying the Chaos commander in the region, a Daemon Prince known as the Horned God. When the Exorcists finally cornered the Daemon Prince in the maze-like alien ruins of the Dead World of Belphago, it was guarded by thousands of fanatical warriors and several warriors of Night Lords Chaos Space Marines it had bound to its will.
What followed was one of the most difficult and costly battles in the Chapter's history, with the Exorcists and accompanying Ordo Malleus strike teams fighting their way through suicidal waves of Cultists and Night Lords before confronting the burning daemons that served the Horned God. In a final titanic struggle, the Exorcists Enochian Guard and a handful of surviving Inquisitors battled the Horned God, who decimated the Daemonhosts of Inquisitor Grey and devoured the Daemonhunter whole. Despite this, the Exorcists refused to retreat and slowly the weight of their attacks began to take their toll on the Daemons.
The turning point in the battle came when one of the Chapter's Venerable Dreadnoughts, Sybra, sacrificed himself to injure the Daemon and allow the rest of the Exorcists to press their attacks. Finally, the Librarian Malachite pierced the Horned God's heart with his force spear, banishing the Daemon. With the loss of their master, the Horned God's hordes were broken. Many who had bartered their souls to the Horned God in exchange for fire simply burst into flames. Fully a third of the Chapter had perished, but within a year the Ork marauders had been defeated and the sub-sector brought back under Imperial control.