The Black Legion
The Black Legion was originally known as the Luna Wolves, the legion of Horus himself. In honour of Horus's great achievements the Emperor decreed that the legion should be renamed the Sons of Horus. Sometime after their defeat and exile, the legion renamed themselves "The Black Legion".
The Space Marines of the Luna Wolves legion were created using human stock taken from the violent hive gangs inhabiting a planet called Cthonia. This planet allegedly existed in one of Earth's closest neighbouring systems. Being within reach even for non-warp spacecraft, Cthonia had been colonised, built upon, tunneled and mined probably since the dawn of space travel. As such, all natural resources had been stripped away and used up millennia before, and the ancient mining technology had long since been rediscovered and removed by the Adepts of Mars. The planet that remained was largely redundant and abandoned, completely riddled with catacombs, crumbling industrial plants and exhausted mine-workings. One explanation given for the legion's name is that the initial batch of recruits were found on Cthonia but taken to the geno-laboratories on Luna for the modification and indoctrination required to become Space Marines.1 Cthonia was so close to Terra that it appears that at least one Cthonian native was able to take part in the Unification Wars and then become one of the first Luna Wolves.
Horus, the Primarch of the Luna Wolves, was the first of the Primarchs to be recovered by the Emperor, having been cast much closer to Terra than the others, and was found at a much younger age. As a result, Horus was for many years the Emperor's only son, and there was a great affinity between them. The Emperor spent much time with his protege, teaching and encouraging him. Horus was soon placed in command of the Luna Wolves legion - ten thousand Space Marines created from his own genetic code. With these warriors to lead, Horus accompanied the Emperor for the first thirty years of the Great Crusade, and together they forged the initial expansion of the young Imperium.1 Horus' own Legion had all the glory of being the greatest Primarch's personal guard, and they shared Horus' credo of fighting to be the best. Under his inspiring command, the Luna Wolves were always at the forefront of the latest campaign, pushing the boundaries of the Imperium ever wider, driving further and further into the galaxy and striving to conquer and liberate more worlds than the other Legions.
At the conclusion of the Ullanor Crusade, the Emperor declared it the greatest victory yet for his mighty Imperium and was said to bestow much praise upon the Luna Wolves and Horus for their part in the campaign. At the subsequent Triumph of Ullanor, the Emperor himself bestowed upon Horus the title of Warmaster, making him the supreme commander of the Emperor's forces. The Emperor also suggested, before he returned to Terra and left the rest of the Crusade to Horus, that Horus should rename his legion to cement his position as Warmaster. The suggested name was the Sons of Horus. Horus initially declined this honour, and his Legion continued as the Luna Wolves for some small amount of time.
Sons of Horus
Increasingly concerned, however, with a belief that some of the other Primarchs and their Legions did not show him and his Wolves enough honour in their roles as the Warmaster and his personal Legion, Horus, at the suggestion of Sanguinius3, eventually took up the offer made to him by the Emperor to change the name and iconography of Legion XVI. Shortly after the Interex campaign the Luna Wolves became the Sons of Horus.
Not long after the change, Horus was wounded on the moon of Davin by Eugen Temba, an old subordinate who was under the influence of the Chaos Power Nurgle.4 Horus recovered in the Temple of the Serpent Lodge, a warrior and healing lodge on the planet. During his convalescence, he took part in the induction ceremony of the lodge. In the days that followed, some of Horus's officers detected a change in his character. It is now presumed that the warrior lodge was in fact a Chaos coven, which somehow managed to ensnare the Warmaster, allegedly due to scheming from Lorgar, Primarch of the Word Bearers. A similar warrior lodge already existed in his own Legion, started after the Luna Wolves' first visit to Davin - this was an example of the Primarch's well-tried practice to develop ties with local populations at work; feral natives were more easily recruited into the Imperial fold when the 'Warriors from the Stars' had become brothers - and it is believed it was subsequently used by the Primarch to aid in the corruption of his Marines. Warrior lodges in other legions under his command were similarly used. Horus' fealty had changed; his Legion eventually came to believe that he was actually possessed by a Daemon. Whether or not this is true, it is certain that he was now allied body and soul to the Powers of Chaos, and he had a new vision for the Imperium with himself at its head.
The majority of the Sons of Horus, already fiercely loyal and proud of their Warmaster, had no hesitation. They quickly renounced their oaths to the Emperor and started to worship Horus and his new gods. The remaining portion of the Legion was betrayed and wiped out by their brothers on the world of Isstvan III, but not before reverting to the use of their original name, the Luna Wolves.2 The only known surviving Sons of Horus loyalist, Iacton Qruze, was not present on Isstvan III, but he similarly reverted to the name and even the iconography of the Luna Wolves after the events known as 'the Flight of the Eisenstein.
Outside the Legion, Horus' corruption spread to every organisation with which he had dealings, including a division of the Adeptus Mechanicus, and from there to the Collegia Titanica and the Legio Cybernetica. The other Primarchs Horus knew like brothers, and was already well practiced at motivating them. Appealing to their pride, martial prowess and courage while playing upon past grudges and favours, the Warmaster gained the loyalty of fully half the Primarchs. The war that followed was the most terrible in the history of the Imperium, and came close to shattering it forever. Space Marines fought Space Marines and Titans fought Titans as Terra was invaded, and the Emperor's Palace itself was besieged and breached. In the end, though, it was Horus who was slain, and with him died the rebellion. It was a traumatic and devastating blow for the Sons of Horus.
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 a world 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. 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, became embroiled in a series of internecine wars triggered by the Emperor's Children, culminating in the destruction of the Sons' fortress. 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 Fabius Bile.
At this point Abaddon, Captain of the 1st Company (himself rumoured to be a clone of Horus), took the position of Warmaster. Under him, the Sons rejected Horus's name and painted their armor black. Incidentally, Abaddon's elite elements of the Legion's 1st Company had already worn black armor.
Abaddon led the new "Black Legion" in a lightning raid, destroying the corpse and the clones of the Warmaster. The Legion fled onboard their remaining space barge into further exile. They have raided the Eye of Terror and the Imperium ever since.
The Legion in its loyalist incarnation was a flexible fighting force that performed well and adapted quickly to almost any combat situation. It was trained to respond sharply and decisively to the tactical orders of its Warmaster and consequently the chain of command within the Legion was very efficient.
With the Primarch as the apex of the legion's order-of-battle, the next step downwards was filled by the Mournival. Although not an official institution, in practice it was this advisory council that helped the Warmaster finalise and execute his strategies. The four Mournival brothers were seen as the senior Captains of the legion. The Luna Wolves/Sons of Horus were considered a large Space Marine Legion and had at least 25 Companies at one point during the Great Crusade. While no maximum headcount for the legion has been given, when the Luna Wolves 10th Company was deployed in entirety during the Whisperhead action on Sixty-Three-Nineteen it numbered approximately 600 marines.3 If 25 companies existed with the same strength, the Legion at one point could have numbered around 15,000 marines. Each Company was commanded by a Captain, but not all Captains commanded a Company: The 1st Company was considered the elite unit in the legion and sported at least two renowned squads led by senior officers, the Justaerin Terminator Squad commanded by Captain Falkus Kibre and Catulan Reaver (Assault) Squad, commanded by Captain Kalus Ekaddon. These squads, and possibly all of 1st Company, wore black-painted armour, in contrast to the white and later pale green of the rest of the legion. Companies contained mixed squad-types, to ensure tactical flexibility. This included fielding Terminator Squads outside of the 1st Company.
The principle tactic of the legion was a decisive surgical assault aimed directly at the command element of the enemy. A compact but hard-hitting force of marines up to several companies strong would compose the initial (and ideally total) thrust of the effort, and was referred to as the Speartip unit.
Within this command structure existed the secretive warrior-lodge, where marines of all ranks and placements could mingle freely and talk openly without having to follow customary rank discipline. Membership was by invite only, and the lodge-members identified each other by the wearing of a silver medallion emblazoned with the Luna Wolf symbol. This system brought individual marines even closer together and increased their bonds of brotherhood and loyalty toward each other, likely increasing combat performance. However, it would eventually aid in the dissolution of the legion's organisation.
The Legion suffered significantly during the early years of exile when it was leaderless, though since Abaddon's overlordship it has regained a sense of discipline and purpose. Horus's favoured doctrine of 'tearing the throat out of the enemy' by eliminating their high command in a swift strike, remains a well-used tactic.
All Black Legion leaders, from the remaining veterans of the Horus Heresy, to the more recently raised, are both cunning and ferocious. The Legion's tactics have been shaped by its lack of numerical strength compared to other Legions, and commanders are adept at using their troops to the best effect.
After the death of Horus, proper structure within the squads and companies disintegrated, and their later dispersal in various spacecraft further fragmented the Legion. Now warbands of virtually any size and composition can be found following Black Legion Champions - ranking officers from older times or newly emerged leaders who have won favour through their violent deeds. At times, such warbands rally together under the banner of a greater Champion or even Abaddon himself, for a major raid or incursion into the hated Imperium. However, loyalty to differing Chaos gods often leads to internal politics and conflict. Possession by Daemons is still considered highly favourable, and many members of the Legion strive for the honour of being hosts.
The overriding belief of the Legion prior to the Warmaster's demise was in the ultimate superiority of Horus and themselves. In continually seeking to prove themselves as the greatest Legion, they did indeed achieve most in terms of sheer numbers of worlds brought into the Imperial fold prior to the Heresy. Their defeat and exile was a crushing blow to the collective ego of the Legion. It has taken all the strength of character of their new commander, Abaddon, to restore the Legion's sense of pride and refocus on their ultimate goal - to overthrow everything which the Emperor of Mankind created.