Warmaster, Lupercal, The Great Betrayer. How did Horus fall so far and so fast to the insidious callings of Chaos, and why did the Galaxy have to burn?
Source: Our Friends at Lexicanum
Unknown to Horus, his brother Lorgar, along with his entire legion of Word Bearers, had secretly fallen to Chaos and conspired to make Horus the Ruinous Powers’ champion against the Emepror. For many years before acting openly, the Word Bearers usurped loyalist hold on their Legions by establishing Warrior Lodges. The full conspiracy finally manifested during a mission on the Feral World Davin, Horus was wounded in battle by a blade of Nurgle wielded by the Chaos-corrupted Eugen Temba. Falling unconscious, under the advice of Word Bearers First Chaplain Erebus, the natives of the world helped to heal him. However, unknown to the desperate Mournival, the healers and Erebus himself, had all since become corrupted by Chaos and were orchestrating the entire series of events. Having his dying body moved to the Serpent Lodge, Horus was soon subjected to an ancient Chaos ritual by the Davin priests, while Erebus entered his mind disguised as the deceased Hastur Sejanus.
This image of Sejanus showed Horus horrifying visions of the future, where the Emperor ruled as a god and had discarded the Primarchs once they had outlived their usefulness. Erebus also told Horus that the Gods of Chaos were peaceful beings with little interest in the Materium, and it was the Emperor that was intent on destroying their realm on his quest for godhood. Most disturbing for Horus, he was told that the Emperor had used the powers of the Warp to create the Primarchs. Horus was shown a vision of the past where all the Primarchs, himself included, were scattered across the Galaxy into a warp vortex. The scattering of the Primarchs, despite being orchestrated by Chaos instead of the Emperor, seemingly confirmed to Horus that the Emperor had used foul Warp-magics in his sons creations. Horus saw the Emperor, who had banned any research into the Warp, as a hypocrite.
Horus during the Heresy
Despite realizing early on that this Sejanus was but an impostor, Horus nonetheless accepted these revelations and his bitterness towards the Emperor, already growing from his fathers isolation and Secret Project on Terra as well as the formation of the Council of Terra (essentially subordinating the Primarchs to mortal humans), finally manifested in outright hostility. The only attempt to stop the conversion of Horus came from his brother Magnus the Red, who entered Horus’ mind but was unable to interfere with the powerful rituals of the Davin Cultists or convince Horus to remain loyal to the Emperor.[2a]Horus dealt with Magnus afterwards by manipulating Leman Russ into launching an all-out purge of Prospero.
After his experience on Davin, Horus agreed to align with the powers of Chaos in order to overthrow the Emperor, which he had become convinced was a corrupt dictator bent on achieving godhood and forsaking his sons in favor of mortal rulers. The Warmaster soon introduced the taint to the Legions under his direct command, converting Angron of the World Eaters, Fulgrim of the Emperor’s Children, and Mortarion of the Death Guard to his cause. Eventually, Horus was also able to secretly recruit Konrad Curze of the Night Lords, Alpharius Omegon of the Alpha Legion, and Perturaboof the Iron Warriors. Horus also converted Fabricator-General of the Adeptus Mechanicus Kelbor-Hal, founding the Dark Mechanicum, as well as many units of the Imperial Army
Horus moved, along with several other secretly traitor legions, to Isstvan III, seemingly in order to suppress a rebellion and reinstate Imperial control. Once he arrived however, he sent down specially selected troops from all four of the legions with him, sending down all those he knew would never join him in open rebellion. Once they were on the planet he proceeded to virus bomb the entire world and killed billions of inhabitants in seconds, along with hundreds of space marines, and a ground war eliminated the remainder. The psychic shock wave from this event was said to be louder than the Astronomican. Horus then redeployed his forces to Isstvan V where he was met by seven Space Marine legions sent to bring him to Terra to face an inquest into his actions on Isstvan III. Four of these seven legions proceeded to rebel against the Emperor. It now became obvious that Horus had massive power as he hunted down the three legions that had stayed loyal. Only a few Space Marines managed to escape his forces and make it back to Terra, and among those killed was Ferrus Manus, Primarch of the Iron Hands. Eventually Horus would mount the skull of Ferrus Manus in his throne room, where he began talking to it in private and lamenting that he must rely on psychotic generals and daemons instead of true, effective strategists like Ferrus had been.
Following a failed assassination attempt by Shadrak Meduson after the Battle of Dwell, Horus regained a lost memory that the Emperor had erased, that of the Emperor’s unlocking of dark powers on Molech. Horus invaded the planet hoping to gain the same abilities as his father. Horus eventually managed to enter the gate, and after an unspecified but extremely long period of time exited the Warp with his powers greatly increased. While time in the Materiumhad only been mere moments, Horus had been within the Warp for so long he had visibly aged. Inside the Warp for what seemed like an eternity, Horus had won a thousand kingdoms, amassed billion of Daemonic followers, and defied Gods. Eventually Horus forcibly acquired the same power the Emperor had gotten, but with his own force of will and without deception like his Father had. However he had refused a place within the Warp, choosing instead to reenter the Materium with his new power to make war on the Emperor. Aboard the Vengeful Spirit, Horus then survives an infiltration by the Knights-Errant led by his former Mournival adviser Garviel Loken. The attempt failed and Horus killed Iacton Qruze before the Knights-Errant escaped.
Now with the powers of a god and maintaining his previously youthful appearance with his new-found abilities, Horus then set a course to fight his way through the Imperium in an attempt to reach Terra and, ultimately, to kill the Emperor and place himself on the throne. After much fighting, death and betrayal (see Battle of Terra for further information), Horus realised he would have to hurry his attacks in order to secure Terra in time for him to set up the defences sufficiently to prevent the arrival of two loyal Space Marine legions which could potentially turn the balance back in the favour of the Emperor. To this end, Horus ordered the void shields of his flagship dropped to entice the Emperor aboard for one final conflict. Somehow, the loyalist forces arrived scattered throughout the ship, and Sanguinius, Primarch of the Blood Angels, appeared before Horus, who offered him riches and power if he denounced his allegiance to the Emperor. Sanguinius refused and attacked. Horus killed him and was found standing over his broken body when the Emperor entered the chamber.
The Emperor and Horus fought with a power that would have eviscerated any mortal man dozens of times over with each blow. The Emperor held back for much of the battle, remembering Horus as his beloved son and not wishing to believe that he had turned so utterly to Chaos. This allowed Horus to inflict crippling mortal wounds on the Emperor, since nothing short of the Emperor’s full power would be sufficient to defeat him. At the critical point in the battle, a lone Adeptus Custode guard entered the room (while others state the lone warrior to be Ollanius Pius, an Imperial Army soldier). Horus flayed him alive with but a look and in that instant the Emperor realised how far his favoured son had fallen. The sacrifice of the Custodian bought the Emperor time to deliver a finishing blow to Horus. With iron resolve, he gathered his full strength and delivered a massive psychic blow that killed Horus almost instantly and obliterated his very soul. In his final moments, the powers of Chaos were driven from him and the Emperor sensed his favoured son’s return to sanity for a fraction of a second before he finally died.
Learn more about the master of the the Lunar Wolves, the man what would betray a Galaxy: Horus Lupercal