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.
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 (who was, inexplicably, still alive, I guess juvenant treatments go a long way) was actually a worshiper of Chaos. If only the Inquisition had done their job and checked the chapters for taint themselves...
He was executed and his relics, 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.