RISE OF THE DARK MECHANICUM
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 Mechanicus Magi who governed Mars, with sporadic outbreaks of espionage and violence being committed against the various forges 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 Mechanicus, had already secretly chosen sides in case of a potential conflict.
Then, with the outset of the Horus Heresy, the Warmaster Horus sent Regulus, an Adeptus Mechanicus representative who had already thrown in his lot with the Warmaster, to Mars to secure the tentative support of the Fabricator-General of the Mechanicum for his dreams of rebellion. Kelbor-Hal was sceptical at first, for the Emperor had been brought to his forge over two centuries earlier, and Kelbor-Hal had been forced to bend the knee to Him. Yet, for two centuries, the ruler of the once-lowly Terran techno-barbarian tribes had made empty promises of an equal role in His grand crusade of conquest, but that vaunted equality had never materialised in the Fabricator-General's view. And the Mechanicum continued to toil in its myriad manufactoria and orbital shipyards across the galaxy's Forge Worlds to provide the Emperor's armies with the needed weapons of war but received nothing for their efforts but platitudes. Kelbor-Hal knew Horus Lupercal was a warrior of vision, but Kelbor-Hal wanted to know what he could offer in addition to platitudes.
Regulus explained to the Fabricator-General that much had happened since the Emperor took His leave of the expeditionary forces after the success of the Ullanor Crusade and the installation of his favourite son Horus as the Warmaster and his proxy in command of the Great Crusade. He assured Kelbor-Hal that alignments had shifted and that new powers emerged from the shadows, offering their aid to those with the strength of vision to heed them. Horus Lupercal was one such individual and he was now assuredly a friend of the Mechanicus, Regulus assuring the Fabricator-General that the Warmaster pledged to return the Martian Empire to its former glory, and furthermore, he swore to withdraw any non-Mechanicum forces from all of the Forge Worlds after the Emperor had been overthrown.
Still, Kelbor-Hal was not convinced. The Warmaster asked much - not just the Fabricator-General's loyalty, nor the manufacturing facilities at his disposal, but Horus also required the utter destruction of any factions loyal to Terra upon Mars before the Warmaster's forces reach the Sol System - and he had asked much of him and the Mechanicus already. They had already delivered, for they had ensured that materials and weapons were priority-tasked to those expeditionary fleets of the Great Crusade that the Warmaster favoured and had delayed shipments to those not aligned with him. But the Mechanicus had no desire to trade one autocrat for another.
To allay the Fabricator-General's misgivings, the Warmaster promised to provide the Mechanicum with the lost secrets of ancient Standard Template Construct technology that had been recovered from the worlds of the recently subjugated Auretian Technocracy by the Warmaster's Sons of Horus Legion.
The Fabricator-General was impressed with the Warmaster's gift and admitted that it was a valuable STC database, but he wanted more. Regulus had anticipated this demand and told Kelbor-Hal that the Warmaster promised to lift all restrictions on research into those technologies like Abominable Intelligence that the Emperor had declared forbidden. To cement the alliance between the Mechanicum and the Warmaster and display the Traitors' seriousness about their cause, Horus had provided Regulus with the protocols required to unlock the infamous Vaults of Moravec, a repository of forbidden knowledge that the Emperor had ordered sealed two centuries earlier after the signing of the Treaty of Mars. Greedily, the Fabricator-General struck the dark bargain, accepting Horus' proposal and willingly joined forces with the Warmaster, assisting the Traitors with all of the technology of Mankind at his disposal.
SCHISM OF MARS
The Vaults of Moravec contained innumerable artefacts of technology that had been fashioned or corrupted by the malign power of Chaos or were in themselves incredibly dangerous, such as Warp-based weapons and ancient artificial intelligences. 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 to the Machine God across the Red Planet as scrap code - Chaos-contaminated digital source code that was infected with an arcane computer virus - infesting the logi-stacks and Cogitator archives of the Mechanicum, causing literal Chaos to emerge in any Cogitator system that was networked to one of its infected counterparts.
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 amongst what would later be called the "Dark Mechanicum" used to marshal their forces, intent on making sure the whole of Mars was firmly under their rule. The Dark Mechanicum also used the tactics of sabotage and assassination in an attempt to eliminate those who were unwilling to join their cause.
This process 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 Dark Mechanicum.
As he still required some pretence 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 pretence 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.
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 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. 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.
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.
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 kilometres and killed billions of people across the planet.
FORLORN HOPE
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, accompanied by four 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. These forges alone produced the bulk of the armour and weapons of the Legiones Astartes, and 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, thirteen companies of the Imperial Army's Saturnine Hoplites advanced on the Traitors' lines surrounding the forge of Ipluvien Maximal. Further south, two companies of Imperial Fists and four regiments of Jovian Grenadiers 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. Eventually, after more than an hour of desperate fighting, Camba-Diaz 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 counter-attacked, pushing Camba-Diaz's Imperial Fists back to the landing fields, the loyalist Legion suffering grievous losses in their fighting retreat.
Sigismund quickly grasped that the Imperial expeditionary force would be unable to hold the forge as planned, but thanks to the aid of loyalist Fabricator-Locum Zagreus Kane, a great deal of munitions had already been secured for transport back to Terra, including nearly twelve-thousand suits of Mark IV Maximus Power Armour and twice as many Astartes weapons. But the Loyalists had 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 sixty god-engines in all.
Sigismund's desire to wreak 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.
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 actions 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.
