In 999.M37, Abaddon the Despoiler launched the 8th Black Crusade of the Forces of Chaos out of the Eye of Terror into Imperial space. During this incursion into the Realms of Man, Abaddon completed a complex ritual of death known as the Skullgather in the name of Tzeentch, the Changer of Ways. On worlds throughout the Segmentum Obscurus, Imperial citizens were slaughtered in precise numbers and esoteric rituals. Only when the Inquisition finally broke the code of damnation was 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 had been saturated in death.
The Black Legion and its Chaotic allies struck out from the Eye of Terror in every direction as the 8th Black Crusade raged across the stars. At first the Imperium could only guess at the Traitor Legion's plans, as some raids accounted for the genocidal extermination of 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 of men, women and children were killed across the Segmentum Obscurus in a variety of ways and in specific numbers, their corpses left in ritualistic patterns. This slaughter 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 ruling Adeptus Mechanicus Tech-magi in the gears of their own manufactoria; with their destruction, the complex sequence of death was complete and the Master of Fates was greatly pleased, providing unknown benefits to the future plots of the Abaddon the Despoiler and the Black Legion.
- Black Legion: A Codex Chaos Space Marine Supplement (Digital Edition), pp. 36-37, 39