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The Schism of Mars was the deadly civil war that erupted on Mars between the Loyalist factions of the ancient Mechanicum and the Traitors of the Dark Mechanicum during the early days of the broader interstellar civil war in 005-006.M31 between the Imperium of Man and the forces of Chaos loyal to the Warmaster Horus.

This great conflagration between opposing Mechanicum forces mirrored the larger conflict that consumed the galaxy during what became known as the Horus Heresy.


The Emperor of Mankind's Great Crusade continued to expand outwards into the galaxy in the early 31st Millennium, bringing untold numbers of human-settled worlds back under the control of a single human authority and ending the isolation that had consumed the human-settled galaxy during the 5,000 Terran years of the Age of Strife. This massive expansion of the Imperium continued unabated until the Emperor's realm encompassed nearly the entire galaxy.

At that time a new and unexpected threat emerged to challenge Mankind's dominance over the galaxy. This was the terrible interstellar rebellion that would be known to later ages as the Horus Heresy. This revolt was instigated and led by the Warmaster Horus, the greatest and most beloved of the Emperor's superhuman, genetically-engineered sons, the Primarchs. The rebellion began with the Warmaster's virus-bombing of those Space Marines whose loyalties to him were suspect on the world of Istvaan III, and soon took hold amongst 9 of the 18 active Space Marine Legions and many of the Titan Legions serving under the command of the Warmaster.

It is not entirely clear how Horus managed to turn such a significant percentage of the armies under his command against the Emperor, but he was known to be a very skilled and persuasive leader who earned immense personal loyalty amongst his subordinates. But even before the opening stages of his planned insurrection began, he knew he would have to secure the support of the Mechanicum and their superior technology and weapons if he was to defeat the Emperor and conquer the galaxy.

Horus won over the loyalty of many of the Mechanicum's Tech-priests after promising them the lost secrets of ancient Standard Template Construct (STC) technology that had been recovered from the worlds of the recently subjugated Auretian Technocracy by his own Sons of Horus Legion.

War Comes to Mars

The climate on Mars was full of discontent during this tumultuous time in the days just before Horus openly declared his rebellion against the Emperor. There were tense relations between the various Mechanicum Magi who governed Mars with sporadic outbreaks of espionage and violence committed against the various forge cities that represented the primary sociopolitical units of Mars. There were even unconfirmed suspicions that the various Titan Legions of the Collegia Titanica, the most potent military forces available to the Mechanicum, had already secretly chosen sides in case of a potential conflict.

Indeed, a large faction of the Martian Mechanicum led by Fabricator-General Kelbor-Hal had come to believe that the Emperor was a false idol and not the true Omnissiah promised by the Machine God, who had yet to reveal himself. More importantly, Kelbor-Hal harboured great resentment against the Emperor for His conversion of Mars into little more than a weapons factory to supply the forces of the Great Crusade and His ban on certain types of technological research, particularly artificial general intelligence.

Tech-priest of the Mechanicum

At the outset of the Horus Heresy, the Warmaster Horus sent Regulus, a Mechanicum representative who had already thrown in his lot with the Warmaster, to Mars to secure the tentative support of Kelbor-Hal. Regulus convinced the Fabricator-General of the Warmaster's resolve to support increased autonomy for the Mechanicum against the autocratic rule of the Emperor.

As a show of his appreciation for the Fabricator-General's support and of the advantages of siding with the Traitors, Horus provided the access codes to Kelbor-Hal that allowed the Tech-priest to open a repository of forbidden knowledge known as the Vaults of Moravec, which the Emperor had ordered sealed two Terran centuries earlier -- for they contained innumerable artefacts of technology that had been fashioned or corrupted by the malign power of Chaos. But their dark bargain was struck, and the Fabricator-General accepted Horus' proposal and joined forces with the Warmaster, assisting the Traitors with all of the technology of Mankind at his disposal.

When this repository was reopened, there was all manner of forbidden arcane knowledge and weaponry that had obviously been tainted by the corrupting influence of Chaos stored within. Soon the corruption spread throughout the forges and temples across the Red Planet as scrap code -- Chaos-contaminated digital source code that was infected with an arcane computer virus -- infested the logi-stacks and Cogitator (computer) archives of the Mechanicum, causing literal Chaos to emerge in any Cogitator system that was networked to one of its infected counterparts.

Kelbor-Hal, Fabricator General of Mars

As a result of the scrapcode infestation, communication across the world and off-planet was severely disrupted and many essential industries were sabotaged. A period of general confusion followed, which the Fabricator-General and his allies used to marshal their forces, intent on making sure the whole of Mars was firmly under their rule.

This proved to be more difficult than they had at first anticipated. Several regions of Mars remained relatively uninfected by the scrapcode, insulated against the assault by their early adoption of a new, far more secure digital information network technology called the Noosphere. Among these regions were the Forges of Koriel Zeth, Ipluvien Maximal, and Fabricator-Locum Zagreus Kane, who would go on to become the leaders of those Tech-priests who chose to remain loyal to the Emperor and resisted being drawn into Kelbor-Hal's so-called "Dark Mechanicum."

While the opening shots had already been fired when the Kaban Machine of Lukas Chrom destroyed a plasma reactor complex belonging to the Loyalist Adept Ipluvien Maximal, it still took a short while before open civil war began.

As he still required some pretense to unleash war against the Loyalists, Kelbor-Hal and his allies attempted to provoke those unwilling to join them, employing various acts of sabotage, assassination and outright aggression to eliminate those who were unwilling to join their cause. One example of these tactics was when the Dark Mechanicum-aligned Legio Mortis attempted to goad the Legio Tempestus into opening fire first by entering Tempestus territory and refusing to answer hails.

Eventually the pretense Kelbor-Hal required was given to him by the Techno-Magi Koriel Zeth, the Mistress of the forge known as Magma City, when she declared she no longer believed the Machine God existed, though she remained loyal to the Emperor and His goals for Mankind, as technology was developed through the application of science and reason, not blind faith.

Princeps Sharaq encounters the insidious scrapcode.

The Dark Mechanicum used her apostasy against the sacred doctrines of the Cult Mechanicus to declare her and her supporters Hereteks and unleash an assault against Magma City, bringing war to Mars as those pledged to Kelbor-Hal and Horus attacked those determined to remain allied to Terra and the Emperor. Magma City would soon become a focal point in the struggle for those among the Mechanicum who remained loyal to the Emperor as the Omnissiah and His Imperium.

Later histories would record that the first blow of the Martian civil war was struck against Magos Mattias Kefra, whose forge in the Sinus Sabaeus region was housed within the Madler Crater. Titans of the Legio Magna marched from the southern Noachis region and within solar minutes had smashed down the gates of Kefra's forge. Howling god-engines daubed in red, orange, yellow and black, and decorated with flaming horned skull devices, ran amok within the high walls of the crater, crushing everything living beneath them and destroying thousands of standard years of accumulated wisdom in a fury of fire.

Vast libraries burned and weapon shops that served the Imperial Army regiments of the Solar Guard were reduced to molten slag as the indiscriminate slaughter continued long into the night, the Magna Legion's trumpeting warhorns sounding like the atavistic screams of primitive savages.

Further north in the Arabian region of Mars, the great god-engine fabrication yards of High Magos Ahotep in the Cassini Crater were struck by a hundred thermonuclear missiles launched from the atomic silos secreted within the isolated peaks and mesas of Nilo Syrtis. The explosions of the forbidden weapons filled the four hundred and fifteen kilometer diameter of the crater with seething nuclear fire, and sent conjoined magma-streaked mushroom clouds soaring nearly seventy kilometres into the sky.

Along the borders of the Lunae Palus and Arcadia regions, what previously had been simply heated debate between the partisans of the Emperor and the Warmaster erupted into outright warfare as Princeps Ulriche of the Death Stalkers unleashed his god-engines upon the fortress of Maxen Vledig's Legio Honorum.

Caught by surprise, the Legio Honorum lost nineteen Titans in the first solar hour of battle, before withdrawing into the frozen wastes of the Mare Boreum and seeking refuge in the dune fields of Olympia Undae. Their calls for reinforcement went unanswered, for all of Mars was tearing itself apart as the plague of war spread across the planet in a raging firestorm.

A Heretek of the Dark Mechanicum

Amid the Athabasca Valles, the war machines of the Legio Ignatum and the Burning Stars Titan Legion fought in bloody close quarters through the teardrop landforms caused by catastrophic flooding in an earlier, ancient age of the Red Planet. Neither force could gain the advantage, nor could either claim victory, so after a night's undignified scrapping, both withdrew to lick their wounds.

At Fabricator-Locum Kane's forge, the Mondus Occulum, the outlying forge Cogitators and logic engines had clogged with corrupt data, howling ghosts of sourceless machine-noise and dangerous code packets of infected algorithms that many of the most advanced protective Aegis Protocols were helpless to defeat. Only Kane’s swift action to shut down the Input/Output highways and the fact that the vast majority of his systems had recently been upgraded to take advantage of Koriel Zeth's revolutionary system of noospheric data transference had spared them the worst of the attack, for an attack it surely had been.

Kelbor-Hal himself soon unleashed his twisted Skitarii and Combat Servitors on the Forges of Ipluvien Maximal. Maximal was able to repel the first wave of attackers, but within solar hours he was totally besieged by unique Ordinatus war engines. At the Herschel impact basin of the Mare Tyrrhenum, 900,000 Skitarii and Protectors clashed in a bloody melee that continued until nearly all were dead.

Dark Mechanicum Tech-Adept Lukas Chrom unleashes the Kaban Machine; a deadly robotic machine that possessed the forbidden technology known as A.I. (Abominable Intelligence)

The single greatest loss of life in a single battle of the conflict took place at the Ismenius Lacus forge city after Dark Mechanicum forces unleashed Virus Bombs on the settlement, killing 14 million people within only solar minutes.

The apocalyptic conflict was not limited to the surface of Mars, and high above in the planet's orbital shipyards Traitor voidships exchanged fire with their Loyalist counterparts after forcing the vessels of the Battlefleet Solar to withdraw. During one of these exchanges, the Ark Mechanicus Mechanicum Glorian was destroyed by Dark Mechanicum Frigates and crashed into the Basilica of the Blessed Algorithm on Mars itself. The crash annihilated millions of square kilometers and killed billions of people across the planet.

Magma City soon became a focal point for the resistance, with both the Legio Tempestus and the Knights of House Taranis coming to Koriel Zeth's aid. Defending against the Chaos-fuelled power of the Dark Mechanicum proved a difficult business and soon not only Zeth's forge, but those of Maximal and even Kane found themselves effectively surrounded and besieged by their enemies.

A vicious battle erupted at Magma City, led by the Legio Mortis and its Imperator-class Titan Aquila Ignis. However the Traitor Titan Legion's attack on Magma City was blunted when lava overran the traitor ranks, and most of the Titans of the Legio Mortis (including the Aquila Ignis) were destroyed by the Legio Tempestus. But despite the victory at Magma City, the situation remained dire for the outnumbered and outgunned Loyalist forces across the planet.

Similar instances of open warfare eventually erupted all across the Red Planet as Martian forces, both civilian and military, fought one another in a deadly and escalating civil war in which the destruction unleashed on Mars mirrored that unfolding in the wider galaxy between Loyalist and Traitor forces.

Forlorn Hope

Sigismund and Zagreus Kane witness the attack on Mondus Occulum

A Legio Mortis Reaver-class Titan clashes with Loyalist Titans from the Legio Tempestus outside Magma City during the Schism of Mars.

For those still loyal to the Emperor on the Red Planet, their salvation arrived in the form of a great Imperial expeditionary fleet. Malcador the Sigillite, the Regent of Terra, charged Rogal Dorn, the Primarch of the Imperial Fists Legion and the Castellan of Terra, with a mission of the most vital importance -- to secure the forges of Mars for the Imperial war effort. Dorn informed the Sigillite that he would send his Legion's Champion and First Captain Sigismund, 4 companies of Imperial Fists and a large force of Imperial Army soldiers to secure the forges of Mars.

Across the northern hemisphere of the Red Planet lay an arc of Loyalist control -- the important munitions manufactoria of Mondus Gamma and Mondus Occulum. Within these two vast industrial complexes the Mechanicum fabricated the weaponry and Power Armour for the Legions of the Space Marines. The forges of Mondus Occulum and Mondus Gamma alone produced the bulk of the armour and weapons of the Legiones Astartes. Dorn intended for his force to strike there to capture those forges and once they were occupied, the Imperial Fists and their allies would push outwards and secure the others.

Sigismund's companies landed at Mondus Occulum as the rest of the Imperial expeditionary force was fighting all across the surface of Mars. After a rapid deployment under fire in the shadow of the caldera of the great volcano Pavonis Mons, 13 companies of the Imperial Army's Saturnine Hoplites advanced on the Traitors' lines surrounding the forge of Ipluvien Maximal. Further south, 2 companies of Imperial Fists and 4 regiments of Jovian Grenadiers (nearly 15,000 Imperial Army soldiers in total) under the command of the Imperial Fists' Captain Camba-Diaz made planetfall in the Mondus Gamma forge complex.

Unfortunately, nothing of the Imperial Fists' mission to Mars went as expected. Camba-Diaz and the Jovian regiments soon became embroiled in a fight for their lives at Mondus Gamma forge, and the Saturnine Hoplite companies tasked with breaking the siege at Ipluvien Maximal's forge were repeatedly turned back by the horrifyingly altered weapon-creatures of the Dark Mechanicum.

Though the fighting was desperate, Camba-Diaz eventually secured the Power Armour forges and the ammunition silos, but his company was outnumbered a hundred to one. The Traitor Magos Lukas Chrom's troops pushed Camba-Diaz's Imperial Fists back to the landing fields and his losses proved grievous. Sigismund soon grasped that the Imperial expeditionary force would not be able to hold the forge as planned, but a great deal of essential supplies had still been secured for transit to Terra.

A traitorous Dark Mechanicum Tech-priest unleashes the destructive capabilities of a forbidden Chaos weapon.

Sigismund's companies had descended upon Mondus Occulum not knowing whether they would have to fight to secure the forge, but it was a relief to discover that Fabricator-Locum Zagreus Kane still held true to the Imperium. Sigismund secured vast quantities of munitions for transport back to Terra, including nearly 12,000 suits of Mark IV Maximus Power Armour and twice as many Astartes weapons.

But the Loyalists had finally run out of time. Despite the Locum's Servitors working at full capacity, it still was not rapid enough, for Sigismund's voidship masters informed him of a sizable enemy force closing in on Mondus Occulum, composed of infantry, armoured vehicles, Skitarii and at least two Traitor Titan Legions comprising nearly 60 god-engines in all.

Sigismund's desire to wreak a bloody vengeance on the heads of those who had rebelled against the Emperor warred with the necessity of the mission his Primarch had given him to secure the vital armour and weapons of Kane's forge that would be needed to defend the Imperial Palace at a later date. Reluctantly he knew he must stay true to his mission, for the forces arrayed against the Imperial strike force were simply too many and his orders did not allow for the pursuit of futile gestures of defiance. Fabricator-Locum Kane warned the Imperial Fists' First Captain that if both the Mondus Occulum and Mondus Gamma forges fell, the Imperium would have no way of replenishing the combat losses the Loyalists sustained in any meaningful way.

Loyalist Mechanicum forces defend the forge of Magma City against the Dark Mechanicum's onslaught

After barely a few solar hours worth of fighting both Mondus Occulum and Mondus Gamma were burning, with great swathes of their industrial machinery and manufacturing capacity destroyed. The loss of such irreplaceable technology and knowledge for all time would be felt by the Imperium for many millennia to come.

Then like comets launching from the surface of Mars, the Imperial landing ships fled for the heavens and their mother ships in orbit. Astartes and Imperial Army vessels jostled in the sky in their haste to depart the Red Planet.

Barely a thousand Loyalist warriors were able to escape from the planet's surface before the manufactoria fell into the hands of the Traitors. The desperate rearguard action of the Astartes and other Loyalists on Mars and the sacrifice of many thousands of lives at least had secured tens of thousands of suits of newly-fabricated Mark IV, Mark V and Mark VI Space Marine Power Armour and other vital materiel that would ultimately prove to be the edge the forces of the Imperium needed in the campaign against the Traitor Legions of Horus Lupercal.

In the aftermath of the battle for Mars, Imperial forces from Terra blockaded the planet, leading to several attempts by the Traitors of Mars to breakout. The Loyalist containment of Mars was overseen by Imperial Fists commander Efried. Ultimately, Mars would remain under the control of the Dark Mechanicum until after the Battle of Terra when the Great Scouring would ultimately drive all of the Traitor forces into refuge within the Eye of Terror.

Across the galaxy, open rebellion erupted as Space Marine Legion fought Space Marine Legion and Titans battled Titans. The traitorous members of what became known as the Dark Mechanicum continued to support Horus at the start of the war later called the Horus Heresy, participating in the horrendous attacks against the Loyalist Space Marine Legions during the Drop Site Massacre on Istvaan V.

Throughout the period of the Heresy they would use dark and forbidden knowledge drawn from the inexhaustible well of Chaos to help destroy the Loyalist forces. Having forsaken the Omnissiah their destinies would be dominated by far darker and more capricious gods.

See Also



Warhammer 40,000 Grim Dark Lore Part 28 – Requiem for a Dream


  • Horus Heresy: Collected Visions (Compilation), pg. 337
  • Mechanicum (Novel) by Graham McNeill
  • Cybernetica (Novel) by Rob Sanders
  • The Horus Heresy Book Six - Retribution (Forge World Series) by Alan Bligh, pg. 30

Battles of the Horus Heresy
004-006.M31 Battle of ProsperoIstvaan III AtrocityWar Within the WebwayBattle of the Somnus CitadelUnrest on CalibanSignus CampaignSchism of MarsBattle of DiamatDrop Site MassacreFirst Battle of ParamarBattle of the Coronid Deeps
007-008.M31 Battle of PhallBattle of RavendelveBattle of the Alaxxes NebulaSiege of the Perfect FortressChondax CampaignSecond Battle of ProsperoFirst Siege of Hydra CordatusBattle of the Furious AbyssBattle of CalthBattle of ArmaturaShadow CrusadePercepton CampaignBattle of IydrisThramas CrusadeFall of Baztel IIIBattle of VannaheimSecond Battle of ParamarBattle of Constanix IISiege of Epsilon-Stranivar IXTreachery at Port MawManachean WarMezoan CampaignBattle of BodtBattle of DwellBattle of Molech
009-010.M31 Xana IncursionCarnage of MoroxSangraal CampaignLiberation of NuminalBattle of ArissakBattle of PerditusBattle of SothaScouring of Gilden's StarBattle of NyrconBattle of TallarnBattle of NocturneBattle of Pluto
011-014.M31 Malagant ConflictBattle of the Kalium GateBattle of CatallusBattle of TralsakTarren SuppressionBalthor Sigma InterventionScouring of the Ollanz ClusterBattle of ZepathBattle of AnuariBattle of PyrrhanSecond Battle of DavinBattle of TrisolianBattle of YarantBattle of KradeBattle of DelugeBattle of Heta-GladiusBattle of the Aragna ChainBattle of KallethBattle of the Diavanos SystemBattle of DesperationBattle of Beta-GarmonDefence of RyzaBattle of ThagriaSolar WarRaid on LunaSiege of TerraGreat Scouring
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Warhammer 40,000 Overview Grim Dark Lore Teaser TrailerPart 1: ExodusPart 2: The Golden AgePart 3: Old NightPart 4: Rise of the EmperorPart 5: UnityPart 6: Lords of MarsPart 7: The Machine GodPart 8: ImperiumPart 9: The Fall of the AeldariPart 10: Gods and DaemonsPart 11: Great Crusade BeginsPart 12: The Son of StrifePart 13: Lost and FoundPart 14: A Thousand SonsPart 15: Bearer of the WordPart 16: The Perfect CityPart 17: Triumph at UllanorPart 18: Return to TerraPart 19: Council of NikaeaPart 20: Serpent in the GardenPart 21: Horus FallingPart 22: TraitorsPart 23: Folly of MagnusPart 24: Dark GambitsPart 25: HeresyPart 26: Flight of the EisensteinPart 27: MassacrePart 28: Requiem for a DreamPart 29: The SiegePart 30: Imperium InvictusPart 31: The Age of RebirthPart 32: The Rise of AbaddonPart 33: Saints and BeastsPart 34: InterregnumPart 35: Age of ApostasyPart 36: The Great DevourerPart 37: The Time of EndingPart 38: The 13th Black CrusadePart 39: ResurrectionPart 40: Indomitus