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The Horus Heresy: Open Rebellion

Horus Heresy P2

The Horus Heresy, also known as the Age of Darkness, was the first and most devastating civil war in Imperial history. Occurring in early 30K and lasting several years, it divided and nearly destroyed the fledgling Imperium. It marked the end of the Great Crusade and the encasing of the Emperor of Mankind into the Golden Throne. Here in part two of our four part series we look at several of the devastating battles that took place across the Imperium.

Battle Of Isstvan III (005-006.M31)

The first unmistakable sign of Horus and his Legion’s descent into Chaos became apparent when Horus initiated a devastating virus bombing campaign on the rebellious world of Isstvan III. The Planetary Governor of Isstvan III had declared independence from the Imperium, leading the Council of Terra to task Horus with the mission of reclaiming the planet. Little did they know that this order would serve to further Horus’ sinister designs. Although the four Legions directly under his command had already turned traitor, there were still some pockets of loyalist elements within the Sons of Horus, World Eaters, Emperor’s Children, and Death Guard. Many of these were Terran Space Marines who had been recruited before their eventual reunification with their respective Primarchs.

Under the guise of following his orders, Horus amassed his troops in the Isstvan System, concealing his true intentions. He had devised a cunning plan to eradicate all traces of loyalty within the Legions under his control. Following an extended period of bombardment, Horus dispatched the loyalist Marines to the surface, ostensibly with the objective of reclaiming the world for the Imperium. However, at the moment when victory appeared at hand, the loyalist Marines were ruthlessly betrayed. Horus issued orders for his ships to unleash a devastating assault on Isstvan III, raining down virus bombs upon the planet’s surface. Nevertheless, some loyalist Marines, devoted to the Emperor, remained aboard their ships and valiantly fought to relay warnings to their comrades on the surface. Their selfless sacrifice managed to save numerous Marines, allowing them to find shelter just before the virus bombs descended. Unfortunately, the population of Isstvan III was not afforded such protection, and the cataclysmic event resulted in the immediate death of twelve billion individuals. The psychic shockwave generated by this mass loss of life reverberated through the Warp, alerting the Emperor to the grave turmoil unfolding and notifying the Chaos Gods that Horus had irrevocably become their pawn.

A group of loyalists, led by Captain Garro of the Death Guard, managed to escape the fleet orbiting Isstvan III aboard the heavily damaged Eisenstein. Their desperate flight aimed to reach Terra and warn the Emperor of the treacherous turn of events.

Upon realising that the virus bombs had not entirely eradicated the loyalist Marines, Angron flew into a fit of rage and launched an assault on the planet with fifty companies of World Eaters. Although Horus was incensed by Angron’s actions for disrupting his plans, he 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 leadership of Saul Tarvitz, fought valiantly against their treacherous battle-brothers, but their cause was ultimately doomed. Gradually, only a few hundred of them persevered until, at last, Horus’ patience waned, compelling him to order Angron to withdraw his forces. He then directed a methodical orbital bombardment that exterminated the last courageous survivors on Isstvan III.

Flight Of The Eisenstein

Led by Captain Garro, a group of seventy loyalists managed to seize control of the Imperial frigate Eisenstein. Evading pursuit by Horus’ forces, they successfully escaped the Isstvan system and entered the Immaterium. However, the Eisenstein had suffered significant damage during its escape from Isstvan III. All of its astropaths had perished, and its sole navigator was gravely wounded. Nonetheless, Garro devised a desperate plan to draw the attention of nearby loyalist ships. He set the vessel’s warp engines to self-destruct and jettisoned them from the ship. Rogal Dorn’s Imperial Fists Legion, which had been stranded in the Warp with its fleet for some time, sensed the detonation of the Eisenstein’s Warp drives and immediately altered course to locate the source. Dorn met with Garro, who proceeded to recount the harrowing events involving the traitorous legions.

The remaining crew of the Eisenstein, now aboard Primarch Dorn’s formidable fortress-monastery, the Phalanx, ultimately made it to Terra. Dorn’s fleet ensured the complete destruction of the Eisenstein to eliminate any potential Chaos taint. This allowed the loyal Marines to report the full extent of the atrocities that had transpired in the Isstvan system. It is said that in the millennia that followed, without this crucial warning, the Imperium would have faced even greater challenges in responding to Horus’ subsequent actions, though some argue that this warning might have inadvertently enabled Horus to carry out the drop site massacre.

The ultimate fate of these seventy loyal Marines remains shrouded in mystery. Some believe they continued to serve the Emperor unwaveringly until their final breaths, while others suggest that they were treated with suspicion, imprisoned, or even executed, viewed as if they were traitorous brethren. There are also those who contend that Captain Garro, profoundly affected by the shocking betrayal, transitioned into the role of an apothecary, vowing never to take another life. Lastly, some speculate that a portion of these individuals formed the core of the esteemed Space Marine Chapter later known as the Grey Knights. This theory is rooted in the fact that Malcador the Sigillite had presented eight of the survivors to the Emperor prior to his departure. These select individuals possessed psychic abilities, hailed from the ranks of the Legions that had turned traitor, yet maintained an unwavering faith in the Emperor and a remarkable resistance to the seductions of Chaos.

Signus Campaign (004-006.M31)

In the Signus Cluster, Sanguinius and the Blood Angels faced a perilous encounter with Daemonic forces led by Kyriss the Perverse and Ka’Bandha, narrowly escaping the clutches of corruption.

Schism Of Mars (005-006.M31)

On Mars, the Warmaster’s allies, the Dark Mechanicum, led by Fabricator-General Kelbor-Hal, initiated the Schism of Mars by unleashing a destructive Chaos scrapcode from the forbidden Vaults of Moravec upon the various forges scattered across the planet. This malevolent act marked the onset of a civil war within the Adeptus Mechanicus.

The unleashed scrapcode wrought havoc, resulting in the devastation of a significant portion of the installations and the disruption of the planet’s communications. Amidst the chaos, the Dark Mechanicum commenced their conquest of the planet. However, they soon discovered that some of the forges had remained relatively intact, thanks to their advanced security systems. The traitorous forces initiated assaults against these pockets of resistance, beginning with the immediate siege of one such Forge led by Ipluvien Maximal. Overwhelmed after putting up a valiant defence, the Tech-Priest chose to detonate the reactors within his own forge to prevent it from falling into the hands of the traitors. In a short span of time, Mars was transformed into a gruesome battlefield, with the relentless battles causing severe damage to the planet’s industrial infrastructure and repositories of knowledge.

Another prominent forge that withstood the initial onslaught of the scrapcode was Magma City, led by the loyal Koriel Zeth. Kelbor-Hal, in his bid to justify the attack on the forge, labelled Koriel a heretic for her refusal to adhere to the Machine God cult. Despite her initial resistance, she eventually succumbed to the overwhelming force of the traitors. Like Maximal, she too made the painful decision to destroy her own forge to deny its use to the enemy.

Following these fierce battles, the loyalist forces received reinforcement in the form of four companies from the Imperial Fists Legion and auxiliary units from the Imperial Army, led by Captains Sigismund and Camba-Diaz. However, given the overwhelming numbers of the Dark Mechanicum forces, it proved impossible to turn the tide of the conflict. Recognising the dire situation, Sigismund ordered the loyal forces to salvage all available munitions, armours, and equipment before evacuating the remaining loyalists from the planet.

The conquest of Mars by the Warmaster’s allies marked a devastating blow for the loyalist forces. Nevertheless, Rogal Dorn, the Primarch of the Imperial Fists, ordered a blockade of the planet to contain the traitors. This blockade would endure until the arrival of the Warmaster’s forces during the Siege of Terra in 014.M31.

The Drop Site Massacre (006.M31)

After purging all suspected loyalist members within the three Legions under his direct command, Horus selected Isstvan V as his central command post, meticulously preparing a treacherous trap for his former brothers and their Legions.

The Emperor, grappling with the agonising betrayal of his beloved son, Horus, issued orders for the deployment of seven entire Space Marine Legions against him. Their directive was clear: apprehend Horus and the Primarchs aligned with him, escort them back to Terra, and demand an explanation for their actions. Unbeknownst to the Emperor, however, four of the Primarchs and their respective Legions chosen for this mission had already secretly turned against him, forming a clandestine “fifth column” poised to strike the loyalists at the most decisive moment.

The initial naval operations unfolded favourably for the loyalists. The Imperial Navy secured orbit over Isstvan V, and the Legions commenced their planetary deployment. Under the overall command of Ferrus Manus, three entire Legions formed the vanguard of the first wave of landings: the Salamanders led by Vulkan, the Raven Guard under Corax, and Manus’ own Iron Hands. Horus possessed advanced intelligence regarding the loyalist drop site, and his forces inflicted significant casualties upon the Legions during their landings, effectively pinning them down and preventing their advance.

A fateful duel between Ferrus Manus and Fulgrim ensued, culminating in Manus’ tragic demise at the hands of his former brother while the Emperor’s Children ruthlessly slaughtered the Iron Hands. The surviving loyalists retreated toward the perceived safety of their brethren in the second wave, hoping to receive reinforcements. However, the four Legions constituting the second wave—the Night Lords under Konrad Curze, the Iron Warriors led by Perturabo, the Word Bearers commanded by Lorgar, and the Alpha Legion of Alpharius—had already succumbed to the seduction of Horus’ cause. They turned their weapons upon their unsuspecting brothers, mercilessly slaughtering them. Following the battle, Fulgrim presented the severed head of Ferrus Manus to Horus as a grisly trophy.

Only a scant number of loyalist Space Marines, bearing the gene-seed of their fallen comrades and carrying the grievously wounded Corax, managed to escape the carnage. They fled back to the Raven Guard’s homeworld, Deliverance, seeking refuge to tend to their injuries and deliver the dire news to the Emperor. In a single devastating stroke, Horus had crippled three entire Legions, slain one of the Primarchs in combat, left another severely wounded, and captured a third, Vulkan, who survived due to his unique status as a Perpetual. Vulkan was subsequently imprisoned by his brother Konrad Curze. This catastrophic turn of events sent shockwaves throughout the Imperium.


Word is now out of Horus’ Betrayal, but the civil war is far from over. In part three we’ll continue to look at the many bloody battles that took place as brother fought against brother. For more in-depth stories about the events of the Horus Heresy look for the Games Workshop Black Library books on our website.