The Abyssal Crusade was an Imperial Crusade undertaken in 321.M37. The judgement of Saint Basillius found thirty Space Marine Chapters lacking faith. The guilty Chapters were given a choice between death or embarking upon a great crusade into the Eye of Terror. All would choose the latter.
After the Ecclesiarchal Purges of the Age of Redemption in 321.M37, the powerful and influential Saint Basillius emerged as an ascendant figure in the Imperium. He even commanded a 'puritas division', several Space Marine Chapters that purged all he commanded. He worked with the Inquisition and Adeptus Arbites across Segmentum Solar, making it mandatory for the populace to inform anyone they suspected as being affiliated to Chaos or Mutants. The few civil liberties in the Imperium were striped back even further, and false allegations were common. However, the level of insurrections in Segmentum Solar dropped almost completely.
When the Warp Storm Dionys appeared, it spread over dozens of systems and levels of mutation and cultist activity rose dramatically. Worse still, those Adeptus Astartes Homeworlds that lay in the path of the Warp Storm had their gene-seed corrupted. When Saint Basillius heard of this, he declared all those Chapters whose homeworlds had become corrupted to be rendered into his judgement. Such was his influence that the High Lords of Terra agreed to the decision, and 30 chapters were found to be corrupted. Basillius ordered that the 30 Chapters take part in a great Crusade into the Eye of Terror, and one by one the Chapters solemnly undertook their quest of certain doom.
The Crusade met with disaster, and each of the Chapters that took part in it had their tale ended in great misfortune. The problems for the Space Marines began immediately upon entering the Eye of Terror, where they were assailed by a Chaos Fleet and scattered. On the world of Anathrax, the Iron Drakes were consumed by the disease-ridden living vegetation and Plague Marines of the world. The surviving members became the Grey Death Chaos Space Marines.
The Sentinels landed on Oliensis, discovering that the entire Daemon World was the form of a morbidly obese man. Ambushed by a force of Noise Marines, they were eventually swallowed up by the planet itself. When they emerged back into reality, they had been corrupted into a cannibalistic Slaaneshi warband known as the Corpus Brethren.
In the Void Galathamar, the Knights Excelsior fought desperately against Daemon Engines on the distorted and disorientating world of Temporia. Eventually managing to escape back to their ships, the Warpsmith Valadrak created an electricity-Daemon and set it loose upon the Knights' fleet. The Machine Spirits of the Space Marine ships rebelled, and with their fleet in disarray the Knights were overrun by pursuing Daemon Engines. Less then a year later, the renegade Magma Hounds emerged.
On the Crone World of Belial IV, the Brothers of the Anvil scoured through the ruins of the original Eldar empire, eventually being captured by Dark Eldar raiders who were scouring the planet for lost technology. After a year of nightmares in the gladiatorial arenas of Commorragh, the broken remains of the Brothers of the Anvil managed to escape from the Dark City. Now insane, they slaughtered all they came across.
One by one, nearly every Chapter that entered the Eye was either destroyed or distorted beyond recognition.
The crusade's mysterious and disastrous fate was kept hidden from the greater Imperium, and the Administratum denied any link to it and the increase of Chaos Space Marine raids shortly afterwards. All that is known is that 800 years after the Crusade began, the remnants of some of the judged Chapters returned from the Warp with their purity and sanity intact. Infuriated and embittered by their ordeal, they cast their sights on the legacy of Saint Basillius, the long-deceased figure now worshiped as a god.
Claiming to have purged more than four hundred worlds within the Eye of Terror, Chapter Master Konvak Lann of the Vorpal Swords would declare the now ancient Saint Basillius a false idol and had in reality been an apostle of Chaos who had engineered the doom of righteous Space Marine Chapters. In less than a year, every known sepulcher and shrine to the "false saint" was destroyed and his followers were massacred. The Saint's bones and relics were then placed in a derelict freighter and launched directly into a nearby star.
- Lectors of Ixis - Became the Oracles of Change
- Knights Excelsior - Became the Magma Hounds
- Invictors - Became The Unhallowed
- Lionguard - Became the Death Shadows
- Graven Fists - Became the Twisted Blades
- Blades Eternal - Fate unknown
- Brothers of the Anvil - Became the Deathmongers
- Sigilites - Became the Malefactors
- Viridian Consuls - Became The Broken
- Altar Brethren - Became the Iconoclasts
- Vengeance Chapter - Became the Flylords
- Sentinels - Became the Corpus Brethren
- Chorus of Eltain - Destroyed
- Tempest Legion - Became The Revelation of Gore
- Bronze Gorgons - Became the Crystal Wyverns
- Prophets of Mercury - Destroyed
- Argent Hammers - Became the Talons of Anathrax
- Iron Drakes - Became the Grey Death
- Illustrians - Became The Black Psalm
- Doom Legion - Became the Vectors of Pox
- Serpents of Light - Became the Predator Legion
- Spears of Olympus - Became the Bloodlords
- Clerics of Steel - Became the Invocators
- Star Gryphons - Became the Sons of Midnight
- Fists of Olchis - Fate unknown
- Justicars - Became The Fractured
- Sanctors of Terra - Became The Blighted Claw
- Crusaders of Dorn - Became the Brotherhood of Lethe
- Vorpal Swords - Portions survived and remained pure
- 1: Warhammer 40,000 6th Edition Rulebook, pgs. 170-171
- 2: Codex: Chaos Space Marines (6th Edition), pgs. 17-19