General Horus was regarded as the finest military commander that the Imperium had produced. His abilities were faultless, and eventually the Emperor granted him the title of Imperial Warmaster. This was a high honour, even in the early years of the Imperium, when brave deeds were commonplace.

Before Horus could travel to Terra to receive his reward he fell ill on the feral world of Davin. This was his undoing. During his convalescence on Davin he was inducted into a secret warrior's lodge, which proved to be little more than a coven. A change of character became evident in the Warmaster - he had been possessed by a Daemon. Horus' membership of the secret lodge was not unusual; Imperial soldiers were often encouraged to join warrior societies of this type. Recruiting was felt to be easier on worlds where 'warriors from the stars' had become 'brothers'.

Warmaster Horus was recalled to duty in preparation for a new Imperial Crusade. It is clear that the Warmaster introduced a system of 'warrior lodges' into the five Legions Astartes Chapters under his direct command. The Chapters were entirely corrupted at the lodges revealed their true nature and showed themselves to be nothing less than Chaos covens. The infection rapidly spread to the Orders of the Adeptus Mechanicus attached to Horus' command. From there the rot spread further into the Imperial forces. More than half of the Adeptus Mechanicus, including many units of the Collegia Titanica and the Legio Cybernetica wholeheartedly supported Horus and his vision of a new Imperium of Chaos. This wholesale treachery went undetected by the Inquisition.

Before Horus could move, the Imperial Commander of Istvaan III declared the entire Istvaan system to be an independent principality. The Emperor and Administratum, ignorant of the change in Horus, his subordinate Chapters and the parts of the Adeptus Mechanicus, ordered the Warmaster to secure the system. Horus chose a bioweapon bombardment on Istvaan III, and the planet became a tomb in seconds. The psychic death scream of the 12 billion who died during the Scouring of Istvaan is reputed to have been louder than the Astronomican.

During the bombardment, loyal Adeptus Astartes officers and troops managed to seize control of the frigate Eisenstein. They had discovered the rot that had been spread through the Warmaster's Chapters and the Adeptus Mechanicus. As Horus completed his withdrawal to Istvaan V the loyalists fled into warp space, carrying a warning to the rest of the Imperium. The seizure of the Eisenstein is regarded as the start of the First Inter-Legionary War...

...The situation grew more desperate by the hour and, when the Outer Palace was abandoned to the Traitor Legions and their allies, the Emperor acted. He disconnected himself from the Astronomican, a signal to the remainder of the Imperial Fleet that the end, one way or another, was approaching. The Emperor and an elite company of Custodes Adeptus soldiery and Imperial Fist Marines were then teleported into Horus' command bunker. In the fierce fighting that followed Horus was killed (although his body was never found) and the Emperor seriously wounded.

The Black Legion is the only rebel Chapter to have changed its name in ten thousand years of exile. Created during the First Founding as the Luna Wolves, the Emperor subsequently changed the Chapter title to the 'Sons of Horus' after the Warmaster's Ullanor Crusade.

It was as the Sons of Horus that the Legion took part in the Horus Heresy, serving as Praetorians during the campaign. However, with the death of Horus at the hands of the Emperor and his Imperial Fist Marines, their morale broke. The sons of Horus fled from the Imperial Palace bearing the remains of the Warmaster, a clear signal that the rebellion had failed. This action alone secured the contempt of all the other Traitor Legions...

...The revived, but still numerically inferior, Sons fought a number of wars against other Traitor Legions, which culminated in the destruction of Sons' Fortress. The Warmaster's body was removed and cloned, much to the disgust of the remaining Sons. Denied their Warmaster, the Sons rejected his name, their Chapter title and painted their armour predominantly black. In a lightning raid the new 'Black Legion' destroyed the Warmaster's body and fled into a further exile...

Source: Realm of Chaos: Slaves to Darkness (1988)

...Horus too, the greatest Primarch of all, was convinced of the virtue of the martial ideals for which he fought...

The leader of the rebellion was the Warmaster Horus, the greatest and most trusted Primarch of all. He had stood by the Emperor's side throughout the long years of the Great Crusade. They had fought back-to-back at the seige of Reillis when the Emperor saved Horus's life. On the battlefield of Gorro, Horus had repaid the debt by hacking the arm from a frenzied Ork as it struggled to choke the Emperor's life out of him.

Horus was the greatest of all Champions of Chaos, an Arch-Champion and Captain of the Great Powers - a Chaos Lord of the highest rank. As the Emperor and his band of warriors materialised inside Horus's Battle Barge they saw for the first time the full extent of the Primarch's treachery. The ship had been transformed into something so horrible that some of the Marines were sent instantly mad...

It took only a few minutes to reach the bridge, though many brave men died in those minutes and hordes of no-longer-human things perished amidst the flames and singing bolt guns. There on the bridge the Emperor confronted his old Warmaster, only to discover Horus poised over the broken body of Sanguinius - the Primarch had found Horus first and had died at his hand.

The Emperor launched his attack, as much a struggle between two old friends as it was a struggle for the fate of humanity. Both knew that whichever of them won would inherit the rule of the galaxy and become the undisputed Emperor of mankind. If Horus won then Chaos would reign supreme and mankind would join the Eldar as a lost race...