The world of Pythos in the Pandorax System of the Demeter Sector was an Imperial Death World, also classified as a Hive World, that was the primary battleground of the Pandorax Campaign fought by the Imperium of Man and the forces of Chaos led by Abaddon the Despoiler in 959-961.M41.
The world became a target for the forces of Chaos because it was home to an ancient subterranean portal to the Warp known as the "Damnation Cache" which dated back to the time of the Horus Heresy. The Imperial forces, which included elements of the Grey Knights and Dark Angels Chapters of Space Marines, ultimately triumphed and drove the Traitor Legions from the Pandorax System.
However, the opening of the Damnation Cache led to the world being transformed into a nightmarish wasteland scoured by daemonic forces and the reality-altering effects of the Empyrean, with great loss of life.
In the Demeter Sector, Abaddon the Despoiler led an alliance of Traitor Legions, including his own Black Legion and the Death Guard, in an invasion of the Imperial Death World of Pythos in the Pandorax System, the location of an ancient hidden gateway to the Warp. Another piece in his dark design to bring an end to the Long War, the Warmaster of Chaos fell upon the world with a small force of Daemon Engines and infernal war machines.
In the space of a few bloody solar days the major hive cities were overrun and the local Planetary Defence Forces crushed beneath the might of the combined Traitor Legions. Only a small contingent of Catachan Jungle Fighters, the CLXXXIII Catachan Regiment of the Imperial Guard, managed to escape the destruction, trekking through the world's inhospitable jungle to the relative safety of the nearest mountain range. Abaddon commenced the complex rituals and sacrifices that would open the Damnation Cache, a portal between realspace and the Warp, breaking open seals that had remained untouched since the time of the Horus Heresy.
Heeding the impassioned cries for salvation from Pythos, the entire Dark Angels Chapter, led by Azrael himself, came to the defenders' aid. Supporting these Space Marines was the Fifth Brotherhood of the Grey Knights led by Supreme Grand Master Kaldor Draigo, dispatched from Titan to deal with the escalating daemonic incursion and the dire peril that the Damnation Cache posed. Both Imperial forces smashed through the Traitor Legion blockade around Pythos, in an epic void battle that pitted the finest warships of the Imperium supported by The Rock, the Dark Angels' mobile fortress-monastery, against the ancient vessels of the Traitor Legions. After the Chaos warfleet was finally driven from orbit and back into the void, the Space Marines were able to send their troops down through the atmosphere, bringing war to its ruined hives and hostile jungles. Failing to allow this distraction to draw him away from the awakening of the Damnation Cache, Abaddon sent an alliance of Legions to hold the Loyalists at bay.
Before either the Dark Angels or the Grey Knights could stop him, Abaddon finally opened the portal to the Warp, daemons spilling out across Pythos. The ensuing conflict devastated the world as reality itself bent and buckled under the raw power of the Empyrean. The unleashed energies created rampant psychic phenomena, and Space Marines fought under boiling skies of blood and in rolling clouds of keening spirits. All across the Demeter Sector, latent psykers were awakened to full and terrible awareness and rebellions sprung up like cancerous boils on dozens of Imperial worlds in the region.
After a long and bitter struggle, a squad of Grey Knights managed to reach the Damnation Cache and close the Warp portal. Starved of their daemonic allies, the Traitor Legions were forced onto the defensive and finally into retreat. Abaddon's forces left Pythos, escaping the vengeance of the Imperium once again, but the world had been irrevocably tainted by the touch of the Warp, its once proud cities and vast wilds twisted into a nightmarish hellscape. Before the portal was closed, however, Abaddon took with him Epimetheus, one of the founding Grey Knights Grand Masters who was a psyker of prodigious strength and the bearer of a unique gift that would aid the Warmaster of Chaos in his forthcoming 13th Black Crusade. This conflict is remembered in Imperial records as the Pandorax Campaign.
- Pandorax (Novel) by C.Z. Dunn