Editing Black Legion (section)

===The Heresy===
After Horus was severely wounded by an anathame on Davin, he received visions from the Chaos gods who told him that the Emperor would cast him and his brother Primarchs aside once he had attained full godhood. After seemingly being easily convinced by them, Horus turned traitor. He corrupted half of his brother Primarchs, the majority of the Imperial army and half of the [[Adeptus Mechanicus]] to his side. He purged his legion of its loyalist elements at the [[Battle of Isstvan III]], sending thousands of his sons down to die. Unfortunately for him, this proved to be a waste of both time and lives that would have been better served striking for Terra, and was therefore arguably Horus' first mistake of the war. Isstvan III was followed by the [[Drop Site Massacre]] on Isstvan V, where the Sons of Horus, Death Guard, World Eaters, Emperor's Children, Iron Warriors, Night Lords, Word Bearers, and Alpha Legion all but exterminated the Iron Hands, Salamanders, and Raven Guard. After this, the Heresy began in earnest. As they so often had been during the Great Crusade, the Sons of Horus were the backbone of the traitor forces. They forced the Manachean Commonwealth and Coronid Deeps into Dark Compliance, seized the Knight World of Molech, burned the Forge World Ohmn-Mat, defeated Shadrak Meduson's Shattered Legions force at Arissak and the Aragna Chain, conquered the fortress world of Beta-Garmon, and all but annihilated the Space Wolves at the Battles of Trisolian and Yarant. The Legion didn't get everything its own way, however. A detachment sent to secure Rangdan weaponry from Advex-Mors was smashed and chased off by the Dark Angels tasked with guarding the xenos artifacts, losing 50 elite Justaerin Terminators and most of the artifacts they had hoped to recover. The Battle of Dwell proved that the Shattered Legions still had fight in them, for Horus was nearly assassinated at its climax. Their homeworld of Cthonia was besieged by the Imperial Fists, severing the pipeline of new recruits from that world and forcing the Sons of Horus to recruit from new worlds. By the Siege of Terra, however, the Sons of Horus believed themselves on the verge of victory. At this point, they were openly worshiping Horus and had big plans on ruling the galaxy. After Horus' defeat, the Sons and their followers fled to the Eye of Terror. Why did they do this instead of, you know, saying "Well Horus is dead but so is your Emperor" and continuing to attack the Imperial Palace where they might have won? Well, there are several reasons. A primarch dying tends to turn his legion from a coherent fighting force into a mob of nigh-suicidal berserkers, as can be seen with the Iron Hands, Blood Angels, and the White Scars (who snapped out of it because the Khan was only mostly dead). Moreover, [[Roboute Guilliman|Rowboat Girlyman]], the Dark Angels, whatever was left of the Space Wolves and Raven Guard, and the Blood Angels going through the Black Rage were hot on their heels (the Iron Hands and Salamanders were still scattered and couldn't do much to help, the Khan was in a coma, and the Scars and Fists were all but bled dry by their losses at the Siege); the whole reason Horus wanted to finish it decisively was to avoid this exact scenario. It of course didn’t help that many of their daemonic allies were banished back to the Warp, leaving them akin to a small, weedy child that had their trousers pulled down on the school playground. It also didn't help that by this point in the siege the Alpha Legion, Emperor's Children, and Iron Warriors had fucked off, the World Eaters were an uncontrollable mob, the Death Guard was in disarray after Mortarion's banishment, and there weren't enough Night Lords, Word Bearers, and Thousand Sons present to be of any consequence. The Sons were the only organized resistance left, their dad was dead, Abaddon was melting down, and every loyalist force in the segmentum was gunning for them.

Really makes you wonder what some of the other Legions were thinking when they blamed the Sons of Horus for losing the war when they barely fought at Terra, but such is the terrible powers of the Ret-Con I guess.