Let’s take a look into the events that led up to the naming of the Black Legion by Abaddon and the painting of their armor!
Via our good friends at Lexicanum
Horus’s death broke the Sons of Horus’ morale, and they were the first of the Traitor Legions to retreat from Terra, an act that would earn them the hatred of all the other Traitor Legions. The Legion re-grouped on the world of Maeleum inside the Eye of Terror. There they built a fortress-tomb for the safe-keeping of the Warmaster’s corpse and even in death still revered him as their commander. Nobody was appointed in his place and the Captains of the Legion would offer sacrifices and pray for guidance in his shrine.
The Sons of Horus were initially the most aggressive Legion against the Imperium, as if to atone for their previous cowardice on Terra or perhaps to attempt to ‘re-prove’ that they were still the best Legion. The Sons dedicated themselves not to one single Chaos Power, but constantly shifted their allegiance to whatever god suited them at the time.
Marines willingly became possessed by the Chaos gods’ daemons; with every change in loyalty, the daemons of the rejected god abandoned the hosts, leaving them lifeless husks. The once great Legion constantly dwindled in number, until they neared extinction. Eventually the desperate experimentation and research by the Legion’s Sorcerer-Librarians uncovered a method of possession that did not destroy the mortal host, saving the Legion.
The Traitor Legions, along with the restored but still numerically inferior Sons of Horus, then became embroiled in a series of internecine wars triggered by the Emperor’s Children legion, culminating in the destruction of the Sons of Horus’ fortress on Maleum.
To the disgust of the Sons of Horus, the Warmaster’s corpse was taken by the Emperor’s Children and several clones were created by their self-styled ‘Primogenitor’, Fabius Bile.
It was at this point that the old Sons of Horus Justaerin Captain Falkus Kibre was able to track down the Vengeful Spirit and Abaddon deep inside the Eye of Terror. Abaddon revealed his intent to destroy the clone of Horus and reform the Sons of Horus into a Legion to finish the war Horus had begun.
After smashing the Emperor’s Children fortress and killing the clone of Horus, Abaddon declared himself the rightful successor of the Warmaster. Painting their armor black, Abaddon renamed his forces the Black Legion, rejecting Horus’ name and all the failure that went with it.
Abaddon managed to reunify the bulk of the old Sons of Horus under his banner, crushing all who resisted. Abaddon gathered many allies across the Eye of Terror, most of the Daemon Primarchs included. Using Chaos rather than being enslaved by it, Abaddon has waged the Long War on the Imperium ever since, most notably during his thirteen Black Crusades.
