The Abyssal Crusade was a massive military campaign undertaken in 321.M37 by the Adeptus Astartes of the Imperium of Man into the hellish realm known as the Eye of Terror located in the Segmentum Obscurus.
The corrupt Imperial Saint Basillius passed judgement and 30 Space Marine Chapters were found wanting. The guilty embarked upon a Penitent Crusade into the Eye of Terror, to purge those worlds stolen by the birth of the Prince of Pleasure, Slaanesh.
They succeeded in battling the forces of Chaos across 400 worlds within the Eye, but suffered horrendous casualties. Thousands of Astartes were slain or corrupted by Chaos.
In the aftermath of the Abyssal Crusade, the number of raids conducted by Chaos Space Marines on worlds in Imperial space increased dramatically.
After the Ecclesiarchal Purges of 321.M37, Saint Basillius the Elder was ascendant within the Imperial Cult. His "Puritas Divisions" of the faithful included several Chapters of Space Marines, each of which was extremely fierce in their vigilance; wherever a mutant or Chaos Cultist was found, an execution would immediately follow, regardless of age, sex, station or circumstance.
For once, the Ecclesiarchy worked in concert with the Inquisition and the Adeptus Astartes across the entire Segmentum Solar, making it mandatory for the populace to inform on anyone they suspected to bear the spoor of Chaos.
Those few civil liberties the Imperial citizenry had left were degraded further, and countless false allegations were made, but the tactic was ultimately effective; the incidence of insurrection against the Emperor dropped to almost nothing.
Tides of Change
This short period of peace did not last, as over a dozen star systems were soon engulfed by Warp Storm Dionys, its echoes rippling along the spiral arms of the galaxy as it raged through the Empyrean.
Many of the Space Marine Chapters with homeworlds affected by the Warp Storm found that the secret imperfections in their gene-seed were writ large upon their new recruits, giving rise to a wave of disturbing manifestations both physical and psychological.
When the Ecclesiarchy heard of this sinister turn of events, Saint Basillius demanded that all those Chapters of the Adeptus Astartes whose homeworlds had been touched by the Warp Storm be rendered unto his judgement. Such was the elder's influence with the High Lords of Terra that within a standard year this had come to pass.
After a series of stringent tests and prognostications, hundreds of Chapters were deemed unaffected by the Warp Storm. No less than thirty were found wanting. "The Judged," as these fallen Chapters came to be known, volunteered to undertake a redemptive Imperial Crusade.
The most militant of their number demanded the right to purify their tainted flesh in the fires of battle, to make a noble end from tragic misfortune. To the surprise of his closest advisors, Saint Basillius agreed to their proposal.
A representative from each of the doomed Chapters held an emergency Council of Dismay to discuss the proposed crusade. After scant solar hours of debate, they acquiesced to Basillius' demands, for they believed that martyrdom was preferable to an existence of suspicion and doubt.
The last few days of 321.M37 saw a solemn procession of strike cruisers and battle barges pass through the Cadian Gate into the Eye of Terror, relay systems dormant and heraldic colours obscured by black mag-plates.
One by one, the Chapters of The Judged disappeared into the iridescent dust nebulae that surrounded the Eye, flickering out of reach from Cadia's astropaths within a single night.
Less than a solar hour after The Judged had made transition into the Eye of Terror, a warfleet of misshapen battle craft encrusted with the sigils of Chaos emerged from the dust clouds to intercept them.
Torpedoes ripped open hulls and blew out docking bays, and Lance fire crackled through the ether to stab through the leviathans of the Imperial fleet. Soon, the battle raged so fiercely that all cohesion amongst The Judged's fleet was lost.
The sheer amount of destruction meted out caused a great turmoil in the Warp, and the fabric of space itself began to tear open. Just as the Chaos warfleet retreated, the ships of The Judged were scattered to the furthest corners of the Eye.
The journeys of The Judged could fill the Great Elucidan Library twice over, each tale with a sorry ending. For one does not venture into the Eye of Terror without courting grievous misfortune.
All that is known, almost 800 standard years after the Abyssal Crusade began, the remnants of some of The Judged Chapters returned from the Warp in 112.M38, having purged more than 400 worlds within the Eye of Terror.
They could barely contain their fury when they were detained by agents of the Inquisition and tested for the taint of Chaos. As they passed every test, a horrible truth came to light.
Saint Basillius still lived, and the testimony of Konvak Lann, Chapter Master of the Vorpal Swords Chapter, revealed him as a false idol, guilty of sending innocents into the jaws of damnation in the service of the Dark Gods.
The Vorpal Swords led the other survivors, blazing with righteous anger, to penetrate the defences of Basillius' palace. They found the unaccountably ancient saint and finally revealed Basillius' true nature as an apostle of Chaos.
The Heretic saint's bones, along with countless relics, books of doctrine and thousands of living worshippers were placed on a derelict bulk-freighter and launched directly into a nearby star, purging all taint of Basillius as if he had never existed.
Even so, the full extent of his corruption and the reach of his influence has never been fully discovered in the millennia since.
Notable Excommunicated Chapters
The following once-Loyalist Chapters of the Adeptus Astartes are known to have been corrupted, in whole or in part, during their time in the Eye of Terror as part of the Abyssal Crusade and to have become Heretic Astartes Renegade Chapters, declared Excommunicate Traitoris by the High Lords of Terra. (Post-Abyssal Crusade, M37 - Appended to the Grimoire Hereticus of M35):
- Codex: Chaos Space Marines (6th Edition), pp. 17-19
- Warhammer 40,000: Rulebook (5th Edition), pg. 125