Legion Wars
Legion wars, also known as Slave wars, was a major conflict between the traitor legions in the Eye of Terror somewhere in M31 (or not, given the fucked-up nature of time in the Eye).
Naturally, after their defeat at Terra, and the Scouring driving them all to the Eye, traitors were quite bitter, and blamed each other for their failures. It didn't boil to the outright warfare, however, until the Emperor's Children had run out of slaves to murderfuck in their orgies, and decided that raiding other legions for their slaves was the brightest idea of how to solve this problem. They were successful at first, especially since they suffered the least in the siege of Terra, but eventually all other legions teamed up and put those bitches in their place. Hard. So hard it's less than a chapter-sized force of of ECs now.
During these wars World Eaters get splintered into small warbands, and Sons of Horus get raped (probably literally) when ECs sacked their base planet and stolen Horus's corpse. This in turn caused Abaddon to leave his emo phase, and start building the Black Legion from what remained of his own legion, along with those he recruited from pretty much every other legion. Newborn Black legion soon attacked EC base of operations and nuked it from orbit with a battlecruiser (by which I mean they THREW a battlecruiser at it), assaulted Fabulous Bile's laboratory ship, killed the clone of Horus, and declared Abby the new Warmaster of Chaos.
After this the war changed from "Emperor's Children versus everyone else" to "Bow to the Despoiler and join his Black Crusades of get rek'd". Some of the legions, like the World Bearers and Iron Warriors happily supported Abbaddon's cause, others, like the Death Guard and Emperor's Children resisted ferociously (some even claim Abbaddon battled and banished Fulgrrim), but most were split, fighting for whatever side they felt like (or whatever side could afford hiring them in case of the Thousand Sons), but in the end the Black Legion and their supporters won, and major fighting amongst the legion in the Eye ceased.