In 301.M39, Abaddon the Despoiler launched his 11th Black Crusade, also called the Doom of Relorria, out of the Eye of Terror. In an attempt to break through the cordon of Warp Storms surrounding the Eye into Imperial space, he employed a captured daemon to navigate.
The effort proved fruitless, and instead the daemon warped the Black Fleet directly into the path of the Orks of WAAAGH! Murgor. In the ruins of the Cardinal World of Relorria, the Black Legion brought the Orks to battle, Bolter rounds and Chainswords tearing into the green-skinned xenos.
After solar months of bloody warfare, Abaddon decided to leave Relorria to its fate and the Black Legion returned to the nightmare realm of the Eye of Terror -- but not before the Warmaster of Chaos filled the holds of his Black Fleet with a multitude of captured Ork Weirdboyz.
In conjunction with a coven of his most powerful Sorcerers, he used the volatile psychic energies of the Greenskin abductees in a daemonic hybridisation ritual that weakened the fabric of reality across the Relorrian System, part of a wider plan to find ways to break down the barriers between the Immaterium and realspace and so aid Chaos' assault on the Imperium.
At the outset of the 11th Black Crusade, Abaddon bound a daemon of Tzeentch within the oculus of his vessel, hoping to find a path through the tumultuous Warp Storms around the Eye of Terror into Imperial space. Tormented with impossible riddles and twisting falsehoods, the daemon was driven completely insane, leading Abaddon thousands of light years off-course. Writhing in its pain, the daemon tore its own substance in a vortex of madness, bending time and space around the fleet.
Abaddon's warships were scattered across the void of realspace. Undaunted by the failure of his daemonic navigator, Abaddon loosed his Black Legion upon the nearest planet, the Imperial Cardinal World of Relorria. Descending on Relorria, the Traitors encountered a rival for their prize -- an Ork WAAAGH! led by the Warlord Murgor "Undred Teef." In a vicious and chaotic three-sided war, daemons and Orks ripped each other apart in the burning ruins of Relorria's vast Reflecting Towers.
Abaddon used this chance encounter with the Orks to conduct unspeakable experiments with daemons and alien creatures, attempting to fuse the material and the immaterial together into some new manner of horror that could be inflicted upon the forces of the Corpse Emperor.
Fascinated by the psychic link between the Orks and their instinctive use of Warp energy, Abaddon took thousands of the green-skinned xenos back with him to the Eye of Terror to continue his research. The Black Legion eventually retreated from Relorria, leaving the battered survivors of humanity to destruction at the hands of the remaining Orks.
- Black Legion: A Codex Chaos Space Marines Supplements (6th Edition) (Digital Edition), "The Doom of Relorria," pg. 40
- Codex Heretic Astartes - Chaos Space Marines (8th Edition), pg. 24
- Liber Chaotica (Background Book), pg. 85