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 up to outright warfare however, until the Emperor's Children had run out of slaves to murderfuck in their rave-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, that their numbers are then less than in a Chapter. (And wouldn't recover until the 13th Black Crusade)
During these wars, the World Eaters got splintered into small warbands and the Sons of Horus got raped (probably literally) when the Emperor's Children sacked Lupercalia, the Sons of Horus' new home planet, and stole 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. The newborn Black legion soon attacked the Emperor's Children 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 Abaddon 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 or get rek'd". Some of the legions, like the Word Bearers and Iron Warriors, happily supported Abaddon's cause, while others like the Death Guard and the Emperor's Children, resisted ferociously (some even claim Abaddon dueled and banished Fulgrim). Most of the chaos forces were split-fighting for whatever side they feel like (or whatever side can afford to hire them, like in the case of the Thousand Sons), but in the end, the Black Legion and their supporters won, and major in-fighting within the Legions in the Eye of Terror ceased.