Wrath of the Chaos Sun
The red giant star at the heart of the Maxil Beta System went supernova, exploding in an expanding cloud of flames covered in Warp energy that spread across nearby systems. The inhabitants of every world for hundreds of light years were exposed to the Warpflame and either turned into Mutants or possessed by Daemons.
In response, the High Lords of Terra ordered the mobilization of whatever forces were close at hand. Grey Knights, Imperial Guard, Space Marine Chapters, and Orders of the Sisters of Battle all responded to the call and purged the Chaos taint. The battle helped to heal the wounds opened by the civil war of the Age of Apostasy.