Battle of Skalathrax
The Battle of Skalathrax is a fairly simple equation: World Eaters + Emperor's Children + sub-absolute-zero weather + limited shelter + Kharn + flamer = "pass the popcorn, this will be interesting" ?. Wrong answer ..... "Awesome!" ? Yes, but try again.... two broken Chaos Space Marine legions..? Correctumundo!
Being servants of rival Chaos Gods, the World Eaters and the Emperor's Children spend much time between Black Crusades fighting each other. One happened on the planet Scalathrax. However, they were on the planet when it stared hitting sub-zero weather that was too much for even Space Marines in Power Armour, not to mention one legion who considers any sensation pleasurable and another legion filled with psychotic berserkers. When the World Eaters decided to take a break from the slaughter, Kharn, Captain of the 8th Assault Company and equerry to Primarch Angron, was furious with his fellow battle-brothers over quitting the field. So Kharn decided to do something about it. Grabbing a flamer, Kharn rampaged around the broken city the World Eaters and Emperor's Children were fighting in, burning every building that could house people and killing everything he foundred. With the number of shelters dwindling, the World Eaters and the Emperor's Children were forced to fight each other for the remaining shelters left. By the time the night was over, both Legions were effectively broken into dozens of marauding warbands.
With the splitting of both legions, the warbands were forced to sell their services as shock troopers for Abaddon or whatever Chaos Lord would put up with them (and trust us, the World Eaters and Emperor's Children are people you don't want to be near). For his part, Fulgrim was content to allow the Emperor's Children to split, and the Emperor's Children now roam the Eye of Terror look for the Pleasure Planet that Fulgrim has been given rule over. Angron, however, was royally pissed off (well, more than usual) and spent several millennia trying to put the World Eaters back together. He finally succeeded with the First War for Armageddon, gathering 50,000 World Eaters (the largest such gathering since Scalathrax) only to have his work undone by the Grey Knights when they banished him back to the Warp. As for Kharn himself, he would go on to earn his legendary epithet "the Betrayer", and now roams the galaxy killing everything that comes his way. Except Abaddon, for some reason.