Long Night of Varonius

The Long Night of Varonius was a brutal and bloody joint-Imperial purgation campaign of retribution conducted by the bellicose and unforgiving Murdering Sons and Sons of the Night Space Marine Chapters against the rebellious population of the Varonius Sector. This two-year long campaign of retribution and slaughter left millions of dead in their wake, and as such, served as a stark reminder to nearby systems of the folly of resisting the rightful rule of the God-Emperor and the High Lords of Terra.

History
The Varonius Sector, a small and near-isolated region of Imperial space, located in the southern fringes of the Segmentum Tempestus, was once a bright star of Imperial majesty and the last vestige of Imperial dignity amidst the growing shadows that long assailed it. Much of the region had become plagued by mysterious "Warp Surges" - smaller, but highly unpredictable manifestation of the much-dreaded Warp Storms, causing the veil of reality and the Immaterium to become thinner over time, which allowed more and more daemonic incursions into the material universe. Attempted rebellions and conspiracies were common and the Imperium usually put most of these insurrections down swiftly and without fanfare. However, occasionally a few rebellions required a different response; not simply subjugation but retribution.

The people of the Varonius Sector did not take easily to the Imperial yoke, as they submitted to the rule of the Emperor in sullen silence. However, over several millennia, as this region of space was subjected to more and more unstable warp activity, this allowed for several hidden Chaos Cults to take root, and bit by bit, they gathered more and more followers and resources, spreading a secret web across the worlds of the sector. The situation grew dire, as several worlds rose up in incurrection and bloody rebellion, refusing to acquiesce to the rightful rule of the God-Emperor and His servants.

When word finally reached the High Lords of Terra of this dire threat, they immediately dispatched the Murdering Sons Chapter to this volatile sector, to enact their dark and savage retribution against the rebellious sector's populace and bring them to heel. Upon their arrival, the Murdering Sons encountered another darkly-famed Chapter of ill-repute - the Sons of the Night. Though both Chapters preferred to operate independently, they found one another of like-mind and similar mein. Therefore, a hasty war council was held between representatives of both Chapters aboard the Murdering Sons' flagship, the Iustus Mortem. Both Chapter Masters laid out their objectives and decided upon which worlds their individual Chapters would launch their campaign.

Once they both came to accord, the Murdering Sons and the Sons of the Night began their own separate campaigns of retribution, bringing bolter and blade to those who foolishly resisted the yoke of Imperial rule. Fearful examples were made, as both Chapters committed horrific atrocities by slaughtering whole populations to serve as a dire warning to those who dared to obey the will of the God-Emperor and those who ruled in His stead. Both Chapters showed no mercy and accepted no surrender.

The carnage unleashed by these two bellicose Chapters were legendary, even by the standards of the Adeptus Astartes, and as such, though this sector was brought back into the rightful rule of the Imperium, the entire region was left in ash and ruin - a charnel house of piled corpses and heaps of skulls - to serve as a stark reminder to the folly of resistance - and further cementing the dark reputations of these two Chapters' use of fear and terror as effective weapons of Imperial retribution and control.