8th Black Crusade

In 999.M37, Abaddon the Despoiler launched the 8th Black Crusade. During this incursion into the Realms of Man, Abaddon completed a complex ritual of death in the name of Tzeentch, the Changer of Ways. On worlds throughout the Segmentum Obscurus, Imperial citizens are slaughtered in precise numbers and esoteric rituals. Only when the Inquisition finally breaks the code of damnation is the Imperium able to bring an end to the Black Legion's rampage across the Segmentum in time with these mysterious attacks, but not before countless worlds have been saturated in death.

History
The Black Legion struck out from the Eye of Terror in every direction as the Eight Black Crusade raged across the stars. At first the Imperium could only guess at the Traitor Legion's plans, as some raids accounted for entire settlements where others only seemed to cause limited carnage by the horrific standards of the Black Legion. On the moons of Teekus, eight of the twenty crater cities were peeled open to the void, their inhabitants freezing in place while others remained untouched. The chartist pilgrim vessel the Divine Path was waylaid en route and 999 of its passengers were murdered in ascending order of the pilgrim tokens they carried. The rest were left to weep over the fallen.

In what later became known as the Skullgather, tens of thousands were killed across the Segmentum Obscurus in a variety of ways and in specific numbers, their corpses left in ritualistic patterns. It was part of a plot to appease the Changer of Ways, the numbers and patterns creating a mathematical equation of terrible and profane perfection. While Imperial forces pursued the Black Legion, trying to bring them to battle, the Inquisition attempted to break the cipher, driving hundreds of adepts insane in the process. Finally, on the Forge World of Rithcarn, the Traitors instigated a massive uprising of the mutant workers, plunging the world into chaos and ruin. In the confusion, the Black Legion sacrificed the council of Tech-Magi in the gears of their own manufactorums; with their destruction, the complex sequence of death was complete and the Master of Fates was greatly pleased.