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 the final part of our four part series we look at the final battles, the siege on Terra and the lasting effects the Horus Heresy had on the Imperium.
The Solar War (010-014.M31)
After enduring years of bitter conflict, Horus and his forces finally reached Terra, the heart of humanity. For several years prior, the traitors had been wresting control of the Sol System from the Imperial Fists in the Solar War. During these battles, the Alpha Legion launched an assault on Pluto, and Alpharius himself was seemingly killed by Rogal Dorn.
The Dissolution of Imperium Secundus (011.M31)
Imperium Secundus faced continuous internal strife, primarily due to political clashes between Guilliman and Lion El’Jonson. Sanguinius, unable to mediate between the two, had visions revealing that the Emperor lived and Terra persisted. These revelations, coupled with the constant disagreements among the ruling triumvirate, led to the dissolution of Imperium Secundus. Although Sanguinius, Roboute Guilliman, and Lion El’Jonson attempted to breach the Ruinstorm and reach Terra, only Sanguinius and the Blood Angels succeeded.
The Battle of Trisolian and the Wolf Cull (012.M31)
Shortly after Sanguinius arrived on Terra, Leman Russ departed with the intention of confronting Horus himself. Thanks to runes placed by infiltrating Knights-Errant during the Battle of Molech, the Space Wolves managed to locate the Vengeful Spirit. Russ engaged Horus in a duel amidst the ongoing battle. Although Russ sustained severe injuries, he managed to wound Horus with the Spear of Russ, purging much of the corruption that had afflicted the Warmaster since Molech. However, this battle exacted a heavy toll on the Space Wolves, who retreated to Yarant with Russ in a comatose state. The Wolves narrowly escaped destruction at the hands of Sons of Horus First Captain Ezekyle Abaddon, thanks to Corax and the Raven Guard.
Battle of Beta-Garmon (012-013.M31)
By this time, Horus’ armada had reached the Sol System and sought to advance toward the Imperium’s Throneworld. To achieve this, they needed to capture the strategically vital Beta-Garmon Cluster. The ensuing Battle of Beta-Garmon, one of the Heresy’s largest conflicts, resulted in massive casualties on both sides. Titan Legions suffered particularly heavy losses, earning the battle the moniker “Titandeath.” Ultimately, Horus’ forces emerged victorious, even though the Warmaster himself succumbed to wounds inflicted by Leman Russ during the Battle of Trisolian.
The Lion’s Purge (012-013.M31)
Emerging from the Ruinstorm, the Dark Angels under Lion El’Jonson embarked on a campaign to purge traitor-held worlds, culminating in the destruction of Chemos, Barbarus, and ultimately Cthonia.
The Muster on Ullanor and Lorgar’s coup (013.M31)
Following the titanic Battle of Beta-Garmon, Horus’ fleet converged on Ullanor for a massive muster of Traitor Primarchs. Despite the injuries he sustained from Leman Russ at the Battle of Trisolian, Horus still led his forces. During this gathering, Lorgar attempted to seize power from Horus, viewing the Warmaster as weak and destined for failure. However, Horus thwarted the coup and banished Lorgar from his presence. Although Alpharius apparently withdrew from the traitor war effort after providing a detailed map of Sol System defences to Horus, the Warmaster still mustered Angron, Magnus, Perturabo, and Fulgrim to his side on Ullanor, along with forces from the Night Lords and Word Bearers. Meanwhile, the Alpha Legion defied Horus’ orders and set out to delay Roboute Guilliman’s advance on Terra.
The Fall of the Death Guard (013-014.M31)
Mortarion and the Death Guard became fully corrupted within the Warp by Nurgle, owing to sabotage by First Captain Calas Typhon. When they re-emerged from the Warp within the Sol System, they had transformed into a horrifying army of Plague Marines.
The Siege of Terra (014.M31)
In the year 0014.M31, on the first day of Primus, Horus’ armada finally reached the Sol System. During the intense final phase of the Solar War, the traitors wrested control of the Solar System from loyalist forces, despite Rogal Dorn’s best efforts to stall until reinforcements arrived from Roboute Guilliman and Lion El’Jonson. Once within range of Terra, Horus and his forces devastated Luna’s naval bases and, within thirty days, overwhelmed the Terran system’s defences. Terra endured bombardment and devastation. The corrupted Astartes from the Sons of Horus, Death Guard, World Eaters, Emperor’s Children, Iron Warriors, Thousand Sons, Word Bearers, and Night Lords eventually landed on Terra, facing fierce resistance from defenders led by the Imperial Fists, Blood Angels, and White Scars. Vast numbers of traitor Imperial Army, Dark Mechanicum, Chaos Cultists, and Daemonic forces joined the traitor Astartes in the assault on Terra. The loyalists were outnumbered, and the battle for Terra escalated into a siege of the Imperial Palace.
By the fifty-fifth day, the rebels reached the walls of the Inner Palace. However, Sanguinius managed to hold the Eternity Gate despite a brutal assault led by the Bloodthirster Ka’Bandha. A stalemate ensued, and not even the siegecraft of Perturabo and the Iron Warriors could break the impasse. At that point, Horus, who had remained in orbit aboard his battle barge, received a warning that the Ultramarines and Dark Angels were returning to Terra and would arrive shortly. If they did, Horus would lose his numerical advantage and face attacks from his rear. In a final gambit, he deactivated the Void Shields protecting the Vengeful Spirit, tempting the Emperor into a decisive confrontation that would determine the war’s outcome.
Leaving Malcador the Sigillite behind to maintain the Golden Throne and thus the seal on the ruptured Imperial Webway, The Emperor seized this opportunity and teleported to the Warmaster’s flagship Vengeful Spirit with Sanguinius and Dorn, Terminator-armoured marines of their Legions, and a number of Custodes. Upon arrival, the Emperor’s forces were scattered throughout the ship and had to fight their way to reunite. Sanguinius encountered Horus first. At the zenith of his powers, augmented by the blessings of all four Chaos Gods and the god-like abilities he acquired during the Battle of Molech, Horus struck down Sanguinius. As the Emperor found Horus, they engaged in a prolonged and gruelling duel. Ultimately, the Emperor succeeded in slaying Horus, but he himself was mortally wounded. The Emperor survived long enough to be transported to the Golden Throne, with Malcador sacrificing his own life to sustain the device in his absence. With Horus defeated, Ezekyle Abaddon, First Captain of the Sons of Horus, took command of the Legion and led a general retreat among the traitor forces. The assault on Terra was repelled, albeit at great cost.
The Emperor’s final command marked the beginning of the Great Scouring. Under the leadership of the remaining loyalist Primarchs, the forces loyal to Horus were relentlessly pursued into the Eye of Terror, and locations like the Isstvan System were purged. All records and the remaining homeworlds of the traitor legions were eradicated. Inside the Eye, the Traitor Primarchs, now nearly all Daemon Princes, became detached from the affairs of the Materium and left their progeny to fend for themselves. Without the Primarchs, the traitor legions descended into a bitter civil war until Ezekyle Abaddon, assuming the epithet “the Despoiler,” emerged as their closest semblance of a unified leader, leading a series of Black Crusades into the Imperium in the millennia to come.
With the Emperor indisposed, Roboute Guilliman assumed the role of Lord Commander of the Imperium and collaborated with his brothers to introduce sweeping reforms aimed at preventing another major rebellion, including the implementation of the Codex Astartes and the division of the Imperial Army into the Imperial Guard and Imperial Navy. The Inquisition and Grey Knights, two agencies left behind by Malcador before his death, played vital roles in maintaining the new order and preventing new heresies. Over the course of the Great Scouring, all remaining loyalist Primarchs either disappeared, were presumed dead, or, in Guilliman’s case, were placed in stasis to evade death due to mortal wounds inflicted by Fulgrim at Thessala.
Ultimately, the Horus Heresy played a pivotal role in the gradual decay of the Imperium over the next ten millennia. The Emperor’s grand ambition, the Imperial Webway, was abandoned, and Chaos continued to plague humanity. The hope and rationality of the Imperial Truth gradually gave way to repression and superstition, with the Imperial Cult now venerating the Emperor as a god, sustained upon the Golden Throne through the daily sacrifice of a thousand Psykers. Technologies from the Great Crusade era were lost over time, and eventually, even the Heresy itself became the subject of myth for the citizens of the Imperium.