Abyssal Crusade

The Fleets of the 30 Judged Chapters on a trip to Hell.

The Abyssal Crusade was a horrible disaster for the Imperium at the beginning of the Age of Redemption. Saint Basillius the Elder had become Ecclesiarch in 321.M37 and immediately started working with the Inquisition and a bunch of Chapters of Space Marines to round up witches, heretics, and mutants, and a good time was had by all.

Well, until a massive warp storm named Dionys came to the party. A ton of Chapter homeworlds were covered by warp rifts and many Chapters found their gene-seed rapidly mutating, which was, of course, heresy. Saint Basillius ordered all of the hundreds of chapters whose worlds had been affected by the storm to come to Terra to be judged. He decided (apparently on advice from his cat) that thirty of the Chapters were CORRUPTED (these Chapters would become known as... the Judged) and declared that they could either crusade into the Eye of Terror or be systematically exterminated. In typical Space Marine fashion, every Chapter chose the former.

To the surprise of nobody (nobody reading Codex: Chaos Space Marines anyway), crusading into the Eye of Terror ended up being an awful, terrible, horrible idea. The thirty Chapters got split up by a Chaos battlefleet almost immediately and landed on all kinds of wonderful Daemon worlds. Most of the Marines either died in horrific ways or were corrupted (for real this time) and became Chaos Space Marine warbands.

Even so, the Vorpal Swords at the very least exterminated four hundred planets in the Eye of Terror. Who knows how many the other Chapters got. It's ironic that this whole thing may have been more successful against Chaos than all the Black Crusades were against the Imperium. If a Chaos fleet hadn't been waiting for them, the combined Astartes fleets might have actually purged most of the Eye.

Eight hundred years later (real time), a few of the Space Marines staggered out of the Warp under Konvak Lann, Chapter Master of the Vorpal Swords, and were understandably pissed. They were immediately detained by the Inquisition and questioned for Chaos taint. When they passed every test, suspicion fell on the one who had judged them in the first place; it turned out that Saint Basillius had actually been a worshiper of Chaos. If only the Inquisition had done their job and checked the chapters for taint themselves...

His relics, remains, followers, scriptures and all that stuff were gathered in freighters and shot into a nearby sun. Nobody remembers him. The only reason the Abyssal Crusade is important, then, is that it caused the creation of hundreds of new Chaos warbands.

Timeline of Warhammer 40,000
The Times of Old Wars of Secession - War in Heaven (60.000.000 BC) - Fall of the Eldar (M30)
Pre-Heresy Age of Terra (M1-M15) - Dark Age of Technology (M15-M25) - Age of Strife (M25-M30)
Great Crusade (M31) The Last Church - Rangdan Xenocides - Interex - Faash - Council of Nikaea
Horus Heresy (M31) Battle of Isstvan III - The Burning of Prospero - Drop Site Massacre - Thramas Crusade
The Battle of Phall - Battle of Calth - Signus Campaign - Imperium Secundus - Siege of Terra
Time of Rebirth (M31-M32) The Great Scouring (M31) - Start of The Long War (M31) - The Legion Wars (M31)
The Battle of Skalathrax (M31) - Creation of the Codex Astartes (M31) - Second Founding (021.M31)
The Forging (M32-M34) The War of The Beast (544.M32-546.M32) - The Beheading (546.M32)
Nova Terra Interregnum (M34-M36) 21st Founding (M36)
Age of Apostasy (M36-37) Plague of Unbelief (310.M36)
Age of Redemption (M37-M38) Abyssal Crusade (321.M37-121.M38)
The Waning (M40-M41) Gothic War (143-151.M41) - The Macharian Crusade (392-399.M41) - The Macharian Heresy (400-470.M41)
Wars for Armageddon (444.M41, 941.M41 and 991.M41) - Damocles Crusade (742.M41)
Time of Ending (M41) The Vaxi Atrocity (731.M41) - First Tyrannic War (745-746.M41) - Sabbat Worlds Crusade (755.M41-780.M41)
Siege of Vraks (813.M41-830.M41) - Massacre at Sanctuary 101 (897.M41) - Badab War (901-912.M41)
The Vaxhallian Genocide (926.M41) - Second Tyrannic War (990.M41-993.M41) - Third Tyrannic War (997.M41-999.M41)
Octarius War (999.M41) - 13th Black Crusade (999.M41-M42)
Age of the Dark Imperium (M42-ongoing) Indomitus Crusade (999.M41-111.M42) - Ultima Founding (M42) - War of Beasts (001.M42-025.M42) - Plague Wars (~111.M42) - Psychic Awakening (M42)