The Black Legion (originally the Luna Wolves) were the 16th Legion of become Space Marines. Their armour was black with white trim before the Heresy and black with gold trim after. Their Primarch was the great Warmaster Horus, who was killed aboard his battle barge to conclude the Heresy. During the Heresy the Sons of Horus developed the unpleasent habit of subsisting only on the livers, kidneys and brains of their slain enemies. Their homeworld was Cthonia, which was later destroyed.
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 honour of its Primarch - so the Legion became the Sons of Horus (circa 125M30).
It was as the Sons of Horus that the Legion fought in the Horus Heresy, serving as his praetorians throughout his campaigns. They were the first to remove the symbol of the Imperial eagle from their armour and banners and replace it with the eye of Horus symbol which became such 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 daemons. 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 armour 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' smouldering hatred of the Imperium and warriors of all the Legions have fought beneath his banner.
Abaddon has marshalled 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 armour. On that day Abaddon will lead his Black Legion and his hordes of the lost and the damned back to the material universe once and for all.
Taken from Codex: Chaos v2
The Black Legion is the only Traitor Legion 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, but it was as the Sons of Horus that the legion fought in the Horus Heresy, serving as praetorians throughout his campaigns. They were the first to remove the symbol of the Imperial eagle from their armour and banners and replace it with the Eye of Horus symbol which became such a dread icon of Chaos to the Imperium. When Horus was defeated, 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.
Although it is very difficult to be sure, it appears that once 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. Although evidence is fragmentary at best, it seems that these internecine conflicts eventually culminated in the destruction of the Sons' final fortress by a combined force of Traitor Legions. To add to the ignominity 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, captian of the first company, as their new Warmaster. Taken from Codex: Chaos Space Marines v3