The Abyssal Crusade was a horrible disaster for the Imperium occurring 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 home worlds 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?
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 (Still alive due to extensive Juvenant treatments) 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 remains, relics, followers, scriptures and everything associated with him was gathered in freighters and shot into a nearby sun. Nobody remembers him. Consequentially, the biggest reason the Abyssal Crusade is important is that it caused the creation of dozens of new Chaos warbands.
This whole thing can be semi-ignored at your leisure, though. Most of the Judged chapters never really show up again after this (notably excepting The Broken, who show up to get wiped out by the Ultrafags), and it's questionable even by 40K's standards of consistency that a Chaos worshipper who daily practices religious rituals to the freaking Anathema wouldn't end up bursting into flames, being purified, exploding, or just not turn to Chaos in the first place.