Hellion War

The Hellion War was an Imperial campaign of reciprocity against a Chaos-infested Hellion Sector in 870.M33. This campaign was carried out by the Dark Angels' Successor Chapter, the Ebon Knights and a mysterious and bellicose Chapter, Imperial scholars believe to be the Murdering Sons, though how this is possible, considering their founding didn't supposedly occur until two millennia later.

History
In the wake of the anarchy that followed the murderous internecine conflict of the War of the False Primarch and its aftermath, the Hellion Sector declares its secession from the troubled Imperium. Bloodily attacking its neighbours, it becomes quickly apparent that the sector's rulers had fallen then to the worship of Chaos, and its millions of men-under-arms were corrupted and given over to darkness, while its mass manufactora were turned to churning forth twisted war machines consecrated to the dark gods. A massive Imperial Crusade is organised and launched to cleanse the Chaos-corrupted sector.

The ever widening war swells to ravage more than eighty inhabited systems and uncounted billions die in the fighting. Extant records indicate that it is during this conflict that the Dark Angels' Successor Chapter, the Ebon Knights, first encountered a mysterious and unfamiliar Chapter of bellicose Astartes that wore sable coloured battle-plate and adorned their armour with fetishes of death. Though not explicitly named, many Imperial scholars believe that this unmentioned Chapter was none other than the darkly-fated Murdering Sons. The Ebon Knights' own Chapter records go on to state that their new-found allies were a bellicose and bloody-handed Chapter that meted out Imperial justice in the most brutal ways possible. The two Chapters found themselves of like-mind and temperament, and began working together during several joint operations, cleansing several recalcitrant worlds and bringing them back into the Imperial fold.

The Ebon Knights continued to valiantly fight for another decade until inexplicably, and without explanation, they pulled their forces out of the conflict. By the dawning of M35, the Hellion Sector and its environs were reduced to little more than a waste zone of barren worlds and dead hive cities. If this account is to be believed, than the known inception of the Murdering Sons predates their known Founding date by nearly two millennia. As of yet, the validity of such claims has yet to be substantiated.