Once open war begins, Mars erupts everywhere. The Dark Mechanicum attacks loyalist forge complexes, libraries, Titan fortresses, and machine-cities. The Magna Legion assaults the Noachis region and reduces its libraries and forges to slag. High Magos Ahotep’s forge-city in the Cassini crater is hit by a hundred nuclear missiles from traitor silos. The Death Stalkers assault the fortress of Maxen Vledig’s Deathbolts, destroying nineteen Titans in the first hour. The Legio Ignatum and the Burning Stars fight to a stalemate at Athabasca Valles. Kelbor-Hal unleashes twisted skitarii and combat servitors against the forges of Ipluvien Maximal, who repels the first wave but is soon surrounded by Ordinatus engines.
This makes the Schism of Mars one of the most brutal battlefields of the early Heresy. Unlike Isstvan III, which is focused on purging loyalist Astartes, Mars is a multi-front industrial apocalypse. Titan Legions fight in their own home territories. Forge-cities burn. Nuclear weapons are used. Libraries vanish. Loyalist and traitor machine cults fight over vaults, factories, data, reactors, and sacred engines.
Magma City becomes the focal point of resistance. Koriel Zeth is aided by Legio Tempestus and House Taranis, the oldest Knight House of Mars. House Taranis fields hundreds of Knight suits at the start of the Heresy and remains loyal to the Imperium, but it is almost completely wiped out fighting the Dark Mechanicum. Only a tiny number of its known pilots, including Raf Maven and Leopold Cronus, survive and go into hiding.
The assault on Magma City is massive. Melgator returns with Regulus, thirteen Legio Mortis Titans, millions of corrupted skitarii, and corrupted machines from the Vaults of Moravec. Legio Tempestus answers Zeth’s call and commits its Titans on Mars to defend the city. This is one of the key Titan/Mechanicum set pieces of the Heresy: loyalist Titans, Knights, and forge defenders against the first true armies of the Dark Mechanicum.
But Magma City cannot hold forever. Zeth refuses to let her forge, her knowledge, or the Akashic Reader fall into traitor hands. In her final act, she opens the sluice gates of the Arsia Mons caldera and disables the safety systems, allowing lava to engulf the city. Magma City dies with its defenders and attackers inside it. This is not a victory in the normal sense; it is denial. Zeth turns her own forge into a tomb so the Dark Mechanicum cannot inherit it.
While the war consumes Mars, Dalia Cythera’s path leads her toward the Noctis Labyrinthus and the hidden truth beneath the planet. She encounters Semyon, Guardian of the Dragon, and learns the forbidden story of the Dragon of Mars and the Emperor’s role in binding it. Semyon passes the role of Guardian to Dalia, and Rho-Mu 31 becomes her protector. Dalia’s fate is not to win the Martian war. Her fate is to guard one of the deepest secrets of Mars for millennia.
This means Mars has two parallel endings. On the surface, the loyalist Mechanicum is crushed and the Dark Mechanicum wins the planet. Beneath the surface, Dalia inherits the hidden duty that predates the Heresy itself. The war decides who controls Mars politically, but the Noctis Labyrinthus plot reminds you that Mars is older and stranger than the Imperium.
As the traitors gain the upper hand, the loyalist Mechanicum cannot fully save Mars. Zagreus Kane, Forge Master of Mondus Occulam and Fabricator-Locum of Mars, refuses to submit to Kelbor-Hal and becomes one of the major loyalist leaders. When it becomes clear that the traitors will win, loyalist forces, including Kane, evacuate the planet with the aid of Captain Sigismund of the Imperial Fists.
After this evacuation, Kane becomes the loyalist Fabricator-General of Mars in exile. This creates the “binary succession”: Kelbor-Hal controls Mars and claims legitimacy as Fabricator-General, while Kane represents the loyalist Mechanicum but does not actually control Mars. Later, Kane’s faction becomes part of the legal and political road toward the Adeptus Mechanicus as a new Imperial Adeptus, but during the Heresy this question remains unstable and bitter.
Mars does not simply fall and disappear from the story. In 005.M31, the early Heresy-era Martian Civil War includes battles at Mondus Gamma and Mondus Occulum, and the Imperial Fists conduct a desperate mission to recover priceless stocks of new-pattern Astartes power armor. The key named Imperial Fists are Sigismund and Camba Diaz. This matters because armor production is not cosmetic in the Heresy. New armor patterns, spare parts, and wargear stores are strategically vital for the Legions fighting across the galaxy.
Mars is next door to Terra. Losing it means the loyalists have a traitor forge world sitting inside the Sol System itself. Dorn therefore has to treat Mars like a loaded gun pointed at the Throneworld. He cannot immediately retake it, but he can blockade it, raid it, restrict it, and prevent Kelbor-Hal from freely supplying the traitor war effort.
By 007.M31, loyalists on Mars are reduced to underground resistance. The surface is dominated by traitor forces, Dark Mechanicum forges, corrupted machines, and Horus-aligned military assets. However, the resistance does not vanish. Malcador the Sigillite secretly supports loyalist operations, and during Corax’s brief return to the Sol System, a Raven Guard decapitation force is sent to Mars during the Perditum Incursion to strike traitor targets and aid the resistance.
This phase is useful for campaign design because it makes Mars feel like occupied territory. The loyalists are not marching in grand armies anymore. They are hiding in tunnels, abandoned data-vaults, dead forge corridors, underhives, sealed transit arteries, and shielded bunkers. The traitors control the major industry, but resistance cells can still assassinate, sabotage, steal data, rescue prisoners, and mark targets for orbital strikes.
By 013.M31, the Heresy is nearing Terra. Dorn launches two coordinated operations in response to a sensor-baffling scrapcode transmission attacking Terra while the Warmaster’s advance guard approaches. One operation attacks Mars itself, led by Aster Crohne of the Blood Angels 94th Company and supported by a cohort of the Legio Cybernetica. The other operation occurs far out in the Sol System’s Oort cloud, where Custodes forces investigate an ancient evil in the darkness.
This shows Mars is still actively dangerous right up to the Siege. It is not just “lost territory.” It continues projecting scrapcode, war machines, Dark Mechanicum research, and strategic pressure against Terra. The Red Planet becomes a traitor industrial fortress trapped inside loyalist-controlled space, while Terra prepares for Horus.
After the traitors take over Mars, the Imperial Fists blockade the planet for years. Kelbor-Hal wins the planet, but he is contained. He cannot fully use Mars the way he wants because Dorn’s defense of the Sol System limits Mars’s ability to send out fleets and materiel. Kelbor-Hal relies on agents and disciples to represent him in Horus’s wider war effort.
During the final stage of the Solar War, Horus’s forces break the loyalist blockade and “liberate” Mars from Imperial containment. That allows Kelbor-Hal to take a more direct role again in the traitor war effort as the Siege of Terra begins. But even then, Kelbor-Hal remains on Mars to oversee reconstruction and muster fleets, rather than personally fighting at the Imperial Palace.
Once Horus is defeated at Terra, Mars is reclaimed by the Imperium. Kelbor-Hal’s Dark Mechanicum is ultimately driven away, and the loyalist Mechanicum/Adeptus Mechanicus survives, but the damage is permanent. The Heresy changes Mars forever. The Mechanicum’s old independence is broken and transformed into the Adeptus Mechanicus structure of the later Imperium. The Dark Mechanicum does not die; it survives as a heretek tradition tied to Chaos, daemon engines, forbidden science, and warp-corrupted technology.
Mars is the Heresy’s industrial betrayal. Davin corrupts Horus’s soul. Isstvan III reveals the betrayal. Isstvan V breaks the loyalist Legions. Calth ruins Ultramar. Signus tries to damn the Blood Angels. Terra decides the war. But Mars decides whether the Imperium’s own machinery serves the Emperor or Horus.
For a campaign, Mars should feel like this:
Political layer: Kelbor-Hal sells Mars to Horus for autonomy, STC knowledge, and forbidden vault access.
Religious layer: Orthodox Mechanicum dogma is used to condemn loyalists like Koriel Zeth, while traitors claim Terra caused the disaster.
Tech-horror layer: Scrapcode turns machines, life support, factories, comms, and war engines against their own masters.
Military layer: Titan Legions, Knight Houses, skitarii cohorts, Ordinatus engines, automata, and corrupted machines fight across forge-cities and volcanic industrial zones.
Secret-lore layer: Beneath all the war, the Dragon of Mars sleeps in the Noctis Labyrinthus, guarded by Dalia Cythera after Semyon passes the burden to her.
In one sentence: Mars is where Horus proves he can steal the Imperium’s weapons, not just its warriors — and where the birth of the Dark Mechanicum turns the Great Crusade’s greatest forge into a poisoned engine aimed at Terra.