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 one of our four part series we look at the events that led to Horus’ corruption.
While the Heresy was sparked by a conspiracy orchestrated by the Chaos forces, it was further fuelled by several contributing factors that drove numerous Legio Astartes towards rebellion. Among these factors, the foremost was the Emperor’s unexpected return to Terra and his subsequent seclusion as he immersed himself in a secretive project, which he withheld from his Primarchs, including his beloved son Horus, who held the title of Warmaster. This apparent abandonment of the Great Crusade, coupled with the Emperor’s reluctance to confide in his own sons, sowed seeds of mistrust, resentment, and disappointment among many of the Primarchs towards their father.
Another factor that played a role was the establishment of an administrative entity called the Council of Terra. Numerous Primarchs perceived these human bureaucrats as encroaching upon their rightful position as leaders of the Imperium they had tirelessly forged. To compound matters, the Primarchs were deliberately excluded from the Council, giving rise to apprehension among many of them about the prospect of an Imperium governed primarily by human administrators, rather than the Emperor’s sons and their Astartes Legions.
Moreover, the Heresy’s catalyst can be attributed to the Emperor’s disciplinary actions against Lorgar and the Word Bearers, setting in motion the chain of events that led to the Heresy itself. Following a public humiliation and severe reprimand of Lorgar and his entire Legion, during which they were compelled to kneel before the Emperor for propagating the belief in his divinity, the devout Word Bearers felt a profound sense of betrayal. Desperate to find a higher power to venerate, they embarked on a quest to seek any formidable force within the universe. This quest eventually led Lorgar and his Legion into the depths of the Eye of Terror, where they pledged their allegiance to the Chaos forces and began plotting against the Emperor. Thus, in secret, the Word Bearers became the first Chaos Space Marines. Covertly scheming to wage war against the Emperor, they clandestinely established Warrior Lodges, overseen by their Chaplains, within the ranks of the other Astartes Legions. While appearing innocuous initially, many of these lodges would later evolve into breeding grounds of support for Horus’ rebellion in the impending conflict.
First Lord of the Imperium, Malcador the Sigillite, suggested that the origins of the Heresy might have been intentionally set into motion by both himself and the Emperor. According to Malcador, they deliberately incited discord among the Primarchs, pitting them against each other, with the subsequent war serving as a means to eliminate certain uncontrollable Primarchs and their respective Space Marine Legions, much like they had done previously with the Thunder Warriors. This strategy aimed to position humanity, rather than bio-engineered superhumans, as the rulers of the galaxy. However, the intervention of the Ruinous Powers disrupted their plan before it could fully materialise, ultimately leading to the catastrophic events of the Horus Heresy. It remains unclear whether this account is partially or entirely untrue, as Malcador himself maintains an air of ambiguity regarding its authenticity.
The Horus Heresy was truly set in motion when Warmaster Horus suffered a grievous injury at the hands of the possessed Eugen Temba, who wielded the stolen Anathame on the ill-fated moon of Davin, a place tainted by the corrupting influence of the Chaos God Nurgle. Despite Horus’ superhuman immune system and the earnest efforts of the Sons of Horus’ most skilled apothecaries, the wound inflicted by the accursed blade refused to heal. In a desperate attempt to find a remedy, the Mournival transported Horus to the Davinite Serpent Lodge, believing it held the key to his recovery. Unbeknownst to all parties involved, Erebus and the Word Bearers had masterminded the events on Davin, orchestrating them in secret.
During the arcane rituals, Horus’ consciousness was transported into the perilous realm of the Warp. There, Erebus, cunningly disguised as Hastur Sejanus, Horus’ closest confidant, unveiled a chilling vision of the future that would result from his actions: an Imperium characterised by repression, violence, and superstition, where the Emperor and some of the Primarchs (excluding Horus) were elevated to divine status by fervent and uninformed masses of humanity. In this fabricated reality, the Chaos Gods portrayed themselves as victims of the Emperor’s overwhelming psychic might, disinterested in material world domination.
Horus, already harbouring deep-seated jealousy and resentment towards his father, the Emperor, readily embraced these false visions. He was among those who dreaded the prospect of a peace settlement that would seemingly squander their hard-fought achievements in favour of weaker individuals, while his legions were cast aside as mere peacekeepers. Thus, Horus proved all too susceptible to the Ruinous Powers’ deceptive promises of an Emperor seeking to ascend to godhood at Horus’ expense.
However, an unforeseen twist awaited them all: Horus’ brother, Magnus the Red, the Primarch of the Thousand Sons, had delved deep into the forbidden arts of sorcery and was determined not to let his brother succumb to the Warp’s temptations. The colossal cyclopean figure materialised within Horus’ hallucination, revealing the true identity of the chaplain and beseeching Horus not to yield to the snares of Chaos. Regrettably, Horus had already made up his mind, convinced that he alone deserved godhood, rather than the Emperor. He accepted the Chaos Gods’ offer to join their cause, in return for which they healed his grievous wound and bestowed upon him the formidable powers of the Warp. The pact between Horus and the Chaos Gods was straightforward: “Deliver us the Emperor, and we shall bestow upon you dominion over the galaxy.”
Swaying The Legions
Renouncing his allegiance to the Emperor, Horus led his Legion into the dark embrace of the Chaos Gods. Horus’ strategic brilliance became evident as he successfully converted half of the Legions, along with numerous regiments from the Imperial Army and several Titan Legions, to his cause. In his eyes, the Emperor was revealed as a man unworthy of the admiration and loyalty of humanity.
The initial trio of Primarchs to stand by the Warmaster included Angron of the World Eaters, whose insatiable love for violence resonated with Khorne; Fulgrim of the Emperor’s Children, who succumbed to the allure of a daemon weapon promising eternal perfection under Slaanesh’s influence; and Mortarion of the Death Guard, whose prior corruption through the efforts of his first captain, Calas Typhon, made him susceptible to persuasion. Erebus had already secured the allegiance of Lorgar and the Word Bearers, aligning these legions with Horus’ cause.
Among the other eventual traitors, Konrad Curze faced disciplinary action from the Emperor for his excessive bloodshed on Nostramo; Alpharius chose to join Horus after an ancient alien cabal prophesied that Horus’ victory would bring about the downfall of the Chaos powers; Perturabo’s bitterness toward Rogal Dorn and his cold demeanour made him a prime target for corruption. Despite amassing the support of numerous legions, Horus recognised that certain Primarchs would remain loyal to the Emperor. Three of the most steadfast Primarchs, Lion El’Jonson of the Dark Angels, Sanguinius of the Blood Angels, and Roboute Guilliman of the Ultramarines, were dispatched on distant missions far from Terra. The Blood Angels were tasked with confronting the daemon-infested Signus Cluster, while the Ultramarines were sent to Calth, where Kor Phaeron later launched an attack against the loyalists with a formidable force of Word Bearers and legions of Chaos cultists. Unbeknownst to the Lion, a rebellion was brewing on his homeworld of Caliban while the majority of his legion was embroiled in conflict against the Gordian League.
The Imperial Fists and White Scars were geographically too close to Terra to contact without raising suspicion, though Horus mistakenly believed that Jaghatai Khan would ultimately join his cause. Just before the Dropsite Massacre, Horus ordered Fulgrim to sway Ferrus Manus to their side, but Fulgrim underestimated the Gorgon’s unwavering loyalty and barely escaped with his life. Fulgrim pledged to present Manus’ head to Horus as compensation.
The remaining Legions—the Raven Guard, Salamanders, Iron Hands, and Space Wolves - remained resolutely loyal to the Emperor, though they would all pay a heavy price in the battles that lay ahead. Beyond the Legions, Horus had already managed to sway Adept Regulus by promising the deployment of STCs acquired during the war against the Auretian Technocracy, securing the support of the Adeptus Mechanicus for the Warmaster’s cause. Additionally, he had corrupted a significant portion of the Imperial Army and Navy to bolster his forces.
Burning Of Prospero (004.M31)
However, Magnus remained an unresolved matter. The Primarch was keenly aware of his brother’s impending treachery and attempted to forewarn the Emperor about the looming betrayal. Realising that he needed to urgently convey this message, Magnus resorted to using his abilities out of desperation and vindication.
In his determined effort to alert the Emperor, Magnus directed a powerful psychic transmission that shattered the Imperial Palace’s psychic defences on Terra. This breach not only obliterated all the protective wards the Emperor had meticulously established but also resulted in the unintended destruction of the Emperor’s clandestine project: a physical gateway intended for the Emperor’s invasion of the Webway, where he would take the battle to the Aeldari.
Magnus’ forceful assault on the wards opened a gateway for the Warp and its myriad of denizens to pour into Terra. Within the City of Sight, the overwhelming surge of uncontrolled psychic energy wrought havoc, annihilating the Choir Primus and shattering nearly every whisperstone. Millions perished as their minds were incinerated or they fell prey to rampaging daemons. Warp storms engulfed entire settlements, and seismic shockwaves levelled structures across the world.
Having previously outlawed the use of sorcery by the Primarchs and refusing to accept that his most cherished and trusted son, Horus, could turn traitor, the Emperor attributed the betrayal to Magnus and his Legion. He issued a directive for the Primarch Leman Russ to lead his Space Wolves Legion in apprehending Magnus. However, Horus successfully persuaded Russ that Magnus posed a significant threat and should not be allowed to return to Terra alive. The Wolves of Fenris descended upon Prospero, leaving destruction in their wake. Betrayed, defeated, and abandoned by his beloved father, Magnus retreated into the Warp, pledging his allegiance to Tzeentch. The Thousand Sons had never initially planned to side with Horus, but the intricate trap laid by the Changer of Ways for the Red Sorcerer’s legion ultimately led them to join the Warmaster’s cause.
Meanwhile, the catastrophe that unfolded on Terra forced the Emperor to confront a new crisis that demanded his full attention. Leading the Custodes and Sisters of Silence, he took charge of this pressing matter, leaving the management of the rebellion to Malcador and Rogal Dorn.
So there you have it, a summary of the beginnings of the Horus Heresy. In part two we’ll look at the opening conflicts that led to an all-out galactic war. For more in-depth stories about the events of the Horus Heresy look for the Games Workshop Black Library books on our website.