|The Horus Heresy truly began after Horus was wounded by the stolen Anathame on the moon of Davin, a place that was cursed by the foul Chaos God Nurgle. The wound caused by the blade refused to heal, despite Horus’ super-enhanced immune system or the efforts of the Sons of Horus’ best apothecaries. While ill, Horus was taken for healing by the Davinites.
But there was one thing no one had counted on: Horus’s brother Magnus the Red, Primarch of the Thousand Sons, had continued to study the forbidden arts of sorcery, and was not about to let his brother fall to the powers of the Warp. The Cyclopean giant appeared within Horus’s vision, revealing the chaplain’s identity and begging Horus not to give in to the temptations of Chaos. Unfortunately, the Cyclopean Primarch’s voice of reason was left unheard. Horus had decided that if anyone deserved to be worshipped as a god it was he, and not the Emperor. He accepted the offer of the Chaos Gods to join their cause and in return they healed his wound and granted him the powers of the Warp. The Chaos Gods’ pact with Horus was simple: “Give us the Emperor and we will give you the galaxy”.
Swaying the Legions
Renouncing his oath to the Emperor, Horus led his Legion into worship of the myriad Chaos Gods. Horus’s genius was revealed as he converted half of the Legions, along with many regiments of the Imperial Army and several Titan Legions to his cause, revealing the Emperor to be as Horus saw him – a man undeserving of the praise and recognition of the Human race.
Angron of the World Eaters, Fulgrim of the Emperor’s Children and Mortarion of the Death Guard were to be the first of the primarchs to side with the Warmaster. Horus found it easy to exploit the Primarchs’ flaws – Angron’s frenzied love of violence was a match for Khorne; Fulgrim was corrupted by a daemon weapon of Slaanesh and its promise of unending perfection; Mortarion, already a close friend of his brother’s, was too easily persuaded, having been turned long before the Heresy through the efforts of his first captain Calas Typhon. Erebus had already vouched for the support of Lorgar and the Word Bearers, and with these legions at his side Horus’s plans began to come together.
Magnus, however, had yet to be dealt with. The Primarch was aware of his brother’s fall, and attempted to forewarn the Emperor of the impending betrayal. However, knowing that he would have to find a means of quickly warning the Emperor, Magnus decided to use his sorcery to deliver the message as an act of both desperation and vindication. The message penetrated the psychic defences of the Imperial Palace on Terra, shattering all the psychic wards the Emperor had placed on the Palace and destroying his secret project: a physic gate by which the Emperor intended to invade the Webway and take battle to the Eldar. Having already outlawed the Primarch’s use of sorcery and refusing to believe that Horus, his most beloved and trusted son, would betray him, the Emperor instead perceived the traitor to be Magnus and his Legion.
The Emperor ordered the Primarch Leman Russ to mobilize his Space Wolves Legion and take Magnus into custody; Horus, however, persuaded his brother Russ that Magnus was a threat and should not return to Terra alive. The Wolves of Fenris descended upon Prospero, destroying all in their path.
Magnus, betrayed, defeated and forsaken by his beloved father, retreated into the Warp and pledged himself to Tzeentch. The Thousand Sons had never planned to join Horus, but the trap that the Changer of Ways had laid for the Red Sorcerer’s legion led them to the Warmaster’s side regardless.
Of the other eventual traitors, Night Haunter was due to face disciplinary action from the Emperor for his excessive bloodshed on Nostramo; Alpharius chose to join Horus after the prophecy of an ancient cabal of aliens was revealed to him that Horus’ victory would cause the downfall of the Chaos powers; and Perturabo’s cold nature and bitterness towards Rogal Dorn made him an easy target for corruption.
Even with so many legions on his side, Horus was still aware that some of his brothers would never join him. Three of the most loyal Primarchs, Lion El’Jonson of the Dark Angels, Sanguinius of the Blood Angels, and Roboute Guilliman of the Ultramarines, were sent on missions far from Terra. The Blood Angels were sent to the daemon-infested Signis Cluster and the Ultramarines to Calth, where Kor Phaeron had amassed a large force of Word Bearers and millions of Chaos cultists. Unbeknownst to the Lion, a rebellion was soon to occur on his homeworld of Caliban.
The Imperial Fists and White Scars were too close to Terra to be contacted without raising suspicion, though Horus believed (mistakenly) that Jaghatai Khan would ultimately take his side. Shortly before the Dropsite Massacre, Horus also ordered Fulgrim to turn Ferrus Manus to their cause, but the Phoenix underestimated the Gorgon’s loyalty and barely escaped alive. Fulgrim promised he would deliver Manus’s head to Horus in recompense.
The remaining Legions – the Raven Guard, Salamanders, Iron Hands and Space Wolves – remained staunchly loyal to the Emperor, though all but the Wolves would pay dearly for it in the battles to come. Beyond the Legions, Horus had already swayed Adept Regulus with promises of the STCs recovered during the war with the Auretian Technocracy, delivering Adeptus Mechanicus support to the Warmaster’s forces, and had corrupted a large portion of the Imperial Guard and Navy.
|Scouring of Isstvan III
The first sign that Horus and his Legion had turned to Chaos was made evident when Horus virus bombed the rebel world of Isstvan III. The Planetary Governor of Isstvan III had declared his independence from the Imperium, and the Council of Terra charged Horus with the retaking of that world. This order merely furthered Horus’ plans. Although the four Legions under his direct command had turned Traitor, there were still some Loyalist elements within the Sons of Horus, World Eaters, and Death Guard; many of these were Terran Space Marines who had been recruited before being reunited with their Primarchs. Horus, under the guise of his orders, amassed his troops in the Isstvan System.
Horus had a plan by which he would destroy all Loyalist elements of the Legions at his command. After a lengthy bombardment, Horus dispatched all Loyalist Marines down to the planet, ostensibly to bring it back into the Imperium. At the moment of victory, however, the Loyalist Marines were betrayed: with a cold snarl of “Let the galaxy burn!”, Horus ordered his ships to open fire on Istvann III, and virus bombs began to rain down on the planet. However, some Marines loyal to the Emperor had remained on board their ships, and as Isstvan III died, these soldiers fought desperately to warn their brethren on the surface. Their sacrifice saved many Space Marines, as they were able to take shelter before the virus bombs struck. The population of Isstvan III received no such protection. Twelve billion people died almost immediately. The psychic shock of so many simultaneous deaths shrieked through the Warp, alerting the Emperor that something was terribly wrong, and informing the Chaos Gods that Horus was now theirs. A contingent of Loyalists led by Captain Garro of the Death Guard escaped the fleet orbiting Isstvan III aboard the damaged vessel called Eisenstein, and fled to Terra to warn the Emperor.
Angron, realizing that the virus bombs had not been fully effective against the Loyalist Marines, flew into a rage and hurled himself at the planet with 50 Companies of Marines. Horus was furious at Angron for delaying his plans, yet reluctantly reinforced him with troops from the Sons of Horus, the Death Guard, and the Emperor’s Children. On Isstvan III, the remaining Loyalists, under the command of Saul Tarvitz, fought bravely against their own traitorous battle-brothers. But their cause was doomed. Soon only a few hundred of them remained until, finally, Horus grew unable to tolerate the delay and forced Angron to withdraw his forces, and ordered a systematic orbital bombardment that killed Isstvan III’s last brave survivors.
Flight of the Eisenstein
The seventy Loyalists led by Captain Garro commandeered the Imperial frigate Eisenstein and evading the forces of Horus, were able to escape from the Isstvan system into the Immaterium. The Eisenstein was badly damaged during its escape from Isstvan III; all its astropaths were dead, and its lone Navigator was mortally wounded. However, Garro managed to attract the attention of passing Loyalist ships by setting the vessel’s warp engines to self-destruct and ejecting them from the ship. Rogal Dorn’s Imperial Fists Legion had been becalmed in the Warp with its fleet for some time, and his Navigators sensed the detonation of the Eisenstein’s Warp drives. Making an immediate course for the location of the ship’s beacon, Dorn met with Garro, who explained to him all that had happened with the Traitor Legions.
The remaining crew of the Eisenstein, now aboard Primarch Dorn’s Fortress-Monastery, the Phalanx, was able to reach Terra (after the destruction of the Eisenstein by Dorn’s fleet to ensure that no Chaos taint was left), allowing the loyal Marines to report the extent of the atrocities that had occurred in the Isstvan system. It was said in later millennia that without this warning, the Imperium would have faced even greater difficulties in responding to Horus’ next moves although his warning may have enabled Horus to enact the drop site massacre.
The fate of these seventy Marines is ultimately unknown. Some believe they continued to fight for the Emperor until death claimed them, while others maintain that they were treated as if they were their traitorous brethren, either imprisoned and left to rot, or executed. Others believe that Captain Garro, shocked by the terrible betrayal, became an Apothecary, vowing never to kill again. Others believe some of these men formed the nucleus of the elite Space Marines Chapter later known as the Grey Knights, for Malcador the Sigillite had presented eight of the survivors to the Emperor before his departure. These men were gifted psykers, came from the ranks of the Legions that had turned Traitor, and yet maintained both an unbreakable faith in the Emperor and talent for resisting the temptations of Chaos.
|Drop Site Massacre
After ridding himself of all suspected Loyalist members within the three Legions under his direct command, Horus chose Isstvan V as his command post and prepared a trap for his former brothers and their Legions.
The Emperor ordered the deployment of seven full Space Marine Legions against him, agonizing over the betrayal of his most beloved son: their orders were to take Horus and the Primarchs with him into custody, then bring them back to Terra to explain their actions. Unknown to the Emperor of Mankind, four of the deployed Primarchs and their Legions had already turned against him, forming a “fifth column” which would strike against the Loyalists at the most decisive moment.
The initial naval operations seemed to go well for the Loyalist side. The Imperial Navy managed gain orbit over Isstvan V and the Legions proceeded with their planetary deployment.
Three whole Legions took part in the first wave of landings, under the overall command of Ferrus Manus and besides his own Legion, the Iron Hands, included the Salamanders lead by Vulkan, and the Raven Guard under Corax. Horus had foreknowledge of the location of the drop site, and his forces mauled the loyalist Legions during their landings, keeping them pinned down at the drop site and unable to advance on the foe.
The four Legions of the second wave – The Night Lords of Konrad Curze, the Iron Warriors of Perturabo, the Word Bearers of Lorgar, and the Alpha Legion of Alpharius – already seduced to the cause of Horus, instead of attacking the rebels, opened fire, slaughtering their unsuspecting brothers in a orgy of carnage widely known as the Istvaan V Drop Site Massacre.
A phrase from the Warmaster himself can easily resume the whole battle: “When the traitor’s hand strikes, it strikes with the strength of a Legion.”
After the battle, the head of Ferrus Manus was delivered by Fulgrim to Horus as a trophy. It is widely believed that Horus planned the battle with aid from the tactical genius, Alpharius.
Only five Loyalist Space Marines, bearing the gene-seed of their fallen brothers and carrying the critically wounded Corax, managed to escape. They fled back to the Raven Guard’s homeworld, Deliverance to tend to their wounds and alert the Emperor what had happened.
In a single stroke, Horus had destroyed three entire loyalist Legions. A loyal Primarch had fallen in battle, another was severely wounded, and a third was missing in action. Vulkan was missing, and how he ultimately managed to survive and to escape is quite unclear. It was disastrous news for the Emperor and the Imperium.
During the developments at Prospero the rebellion would be further be strengthened by Magnus the Red and his Legion, the Thousand Sons, servants of Tzeentch. With nine Legions and much of the Adeptus Mechanicus behind him, Horus quickly struck towards Terra.
|The Siege of Terra and the Aftermath of the Heresy
The Traitor Marines laid waste to their former allies’ holdings along the way to Terra. Throughout the Imperium, rebel and loyal forces battled viciously. Horus and his forces destroyed Luna’s naval bases, and within 30 days had destroyed the Terran system’s defences. Terra was bombarded and devastated, and eventually, the corrupted Marines landed on Terra, experiencing heavy resistance from the forces defending Terra (among these loyalists were Blood Angels, Imperial Fists and White Scars). The loyalists were outnumbered, and the battle for Terra turned into a siege of the Imperial Palace. By the 55th day the rebels had reached the walls of the Inner Palace. Horus, who had remained in orbit aboard his battle barge, was at that point warned that the rest of the loyalist Legions were returning to Terra and would arrive in hours. The Ultramarines were pushing their way to Terra, along with the Space Wolves and the Dark Angels. If this were to happen, Horus’ advantage in numbers would be diminished. He let down the force field protecting his battle barge, to lure the Emperor in a final battle that would decide the war.
The Emperor saw this opportunity and teleported to the Warmaster’s battle barge with two of his Primarchs, Terminator armoured Marines of their Legion as well as members of the Custodes.
After the death of Horus the heresy broke apart, and the remaining Chaos Space Marines retreated into the Eye of Terror, from which they periodically launch Black Crusades into the Imperium led by Abaddon the Despoiler, who is rumoured to be the clone-son of Horus.