The Black Legion began life as The Luna Wolves, created during The First Founding. The Emperor later changed its name to The Sons of Horus, in honour of its Primarch, in recognition of his and its many accomplishments in The Ullanor Crusade. It was as The Sons of Horus that the Legion fought in the Horus Heresy, serving as Horus's Praetorians throughout his campaign. They were the first to remove the Imperial Eagle from their armour and Banners, and replaced it with The Eye of Horus. Factions of the other Traitor Legions later blamed The Sons of Horus for causing the rout from Earth by retreating into Warpspace with the body of their beloved Warmaster. The Sons of Horus at first worshipped one Chaos power after another, more and more of their number becoming possessed by Daemons. Over the centuries, they were also thinned out by their constant wars with the other Traitor Legions and Chaos forces. When Abbadon came to power in the Legion as their new Warmaster, his first edicts rejected the name of Horus and their ancient Legion title. He ordered the remaining members of the Legion to repaint their armour black in eternal memory of their shame at allowing the body of the Warmaster to be taken. He then led then in a lightning raid which destroyed the Warmaster's body and all of his clones. The Legion's remaining battle barge acts as a secret base for Abaddon and his Black Legion from which they make raids into the Imperium and The Eye of Terror.
Codex Chaos 2nd ed.
The Black Legion is the only one of the Traitor Legions to have changed its name in its ten thousand years of exile. The Legion was originally created in the first founding as the Luna Wolves. The Emperor subsequently bestowed a new name on the Legion in recognition of its accomplishments in the Ullanor Crusade, renaming it in honor of its Primarch - so the Legion became the Sons of Horus. It was as the Sons of Horus that the Legion fought in the campaigns. They were the first to remove the symbol of the Imperial eagle from their armor and banners and replace it with the eye of Horus symbol which became a dread icon of Chaos to the Imperium. They fought with pride and unthinkable ferocity, knowing that they were the chosen amongst the Warmaster's new order. When Horus was defeated by the Emperor the Legion's morale was shattered; their patron, their father, was gone. Factions in the other Traitor Legions later blamed the Sons of Horus for beginning the rout from Earth by retreating into warp space with the body of their beloved Warmaster, leaving the horde leaderless below. But the battle for Earth had been lost when Horus fell, and no power in the universe could have brought victory to the forces of Chaos. In the Eye of Terror the Sons of Horus fought ferocious battles with the other Legions for possession of key worlds and resources, fighting to re-establish themselves as the prime Legion. The Sons of Horus worshipped one Chaos Power after another and each time more of their number gave themselves up to possession by demons. Over centuries they were also bled white in a succession of bloody conflicts with different Legions. These internecine conflicts eventually culminated in the destruction of the Sons' final fortress by a combined force of Traitor Legions. To add to the ignominy the Warmaster's body was removed and cloned, much to the disgust of the remaining Sons. Denied their Warmaster, the Sons of Horus swore allegiance to Abaddon, captain of the 1st Company, as their new Warmaster. Abaddon's first edicts rejected the name of Horus and their ancient Legion title. He ordered the remaining Chaos Space Marines to repaint their armor black in eternal memory of their shame and then led them in a lightning raid, which destroyed the Warmaster's body and his clones. The Legion's remaining battle barge disappeared into the gigantic dust nebulae at the edge of the Eye of Terror to serve as a hidden base for Abaddon and his "Black Legion". The Black Legion has raided into the Imperium and the Eye of Terror ever since. Their battle barge has been seen in many parts of the galaxy, only to disappear as mysteriously as it arrived. Abaddon has fought to rebuild the pride and reputation of the Black Legion, always leading his forces into the most dangerous conflicts personally. At first Abaddon won the grudging respect of the other Traitor Legions, but as his deeds have grown mightier he has succeeded in winning their support too. His impassioned words have rekindled the Traitor Legions' smoldering hatred of the Imperium and warriors of all the Legions have fought beneath his banner. Abaddon has marshaled his strength with care and now commands the loyalty of captains from all of the other Traitor Legions. Those who oppose him are crushed; those who join him add their strength to the greatest army ever assembled in the Eye of Terror. Abaddon has tested the strength of the Imperium many times in the Long War, and with each victory his power grows. One day the Imperium will show a weakness, a chink in its armor. On that day Abaddon will lead his Black Legion and his hordes of the lost and them damned back to the material universe once and for all.
