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Underground War

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Underground War
Word Bearers and Ultramarines in battle during the Underground War
Conflict Horus Heresy
Date 007-014.M31[1]
Location Calth, Veridia System, Ultramar
Outcome Pyrrhic Loyalist victory
Traitors[3a] Imperium[3a]
Dark Apostle Maloq Kartho[8c]
Captain Hol Beloth(KIA)[8c]
Dark Apostle Kurtha Sedd(KIA)[5]
Consul-Chaplain Ankarion(KIA)[7c]
Captain Foedral Fell(KIA)[8a]
Captain Remus Ventanus[2b]
Captain Steloc Aethon[5]
Captain Vultius[6]
Captain Valius Thesian(KIA)[7]
Sergeant Aeonid Thiel[6]
Archmagos Cronn Barbarel[3d]
Almost all Traitor forces killed
Anchorite surrendered[4]

The Underground War was a major battle of the Horus Heresy, taking place after the Battle of Calth's main phase. The remaining ground troops at Calth, isolated from the wider civil war and forced underground due to Calth's toxic sun, fought each other for survival from 007 to 014.M31.[1][7c]


The Battle of Calth took place in 007.M31, as the Word Bearers treasonly struck at the Ultramarines in an attempt to weaken or even destroy the loyalist legion before it could intervene in the Horus Heresy. Even though the Word Bearers, supported by a great number of Chaos Cults, Dark Mechanicum forces, and other Traitor elements, succeeded in inflicting massive losses on the Ultramarines, the latter rallied quicker and more decisively than expected. They counter-attacked and the tide of war began to turn.[7a]

However, the Traitors used the powers of the Warp to afflict the sun of the Veridia System, Veridia. Although the Ultramarines prevented the sun from going nova, the star was irreversibly poisoned and changed, and began to bombard Calth with lethal radiation.[2a][2b][6] The Traitors subsequently withdrew, but left most of their ground forces behind; Primarch Lorgar had deliberately selected those Word Bearers for the invasion whom he considered unreliable or replacable. Only some elements belonging to the Graven Star and Unspeaking were evacuated, while the other Word Bearers were left to die on Calth.[3c] In contrast, Primarch Roboute Guilliman ordered an evacuation of the system as complete as was possible in the short time they had left to chase after the Word Bearers. The Ultramarines and other Loyalists contingents left on Calth were ordered to seek shelter in Calth's gargantuan cavern system, though Guilliman promised to eventually return and extract them.[2a][2b][3b]

The remaining Loyalists on Calth rallied under the leadership of Captain Remus Ventanus who went underground at Mark 23:43.00.[2a][2b][3b] The Ultramarines also took as many civilian survivors with them as possible. Calth naturally possessed massive subterranean caverns which had further expanded into so-called "arcologies", areas designed to act as cities for future population growth. Thus, there was enough space to harbor entire armies beneath Calth's now-toxic surface.[7a] Loyalist Mechanicum troops, mostly the remnants of Archmagos Cronn Barbarel's Taghmata Omnissiah, joined the Ultramarines in the deeps. As they were left less affected by the radiation, the Mechanicum contingents helped to cover the retreat into the underground.[3d] The remaining Word Bearers followed the Loyalists into the caverns, beginning the Underground War.[2a][2b][7b]


Early clashes

The Word Bearers and their remaining allies were quick in conquering a number of arcologies, turning them into fortified holdouts.[7b] Other Word Bearers groups would operate as guerrillas, moving among the caverns to ambush loyalists.[6] Either way, the remaining traitors were even more cut off from aid than the loyalists, so they began to scavenge for essential supplies to continue their war, and also sallied out to enact what they believed to be the desires of the Chaos Gods.[7b] Contact among the scattered Loyalist and Traitor groups often proved difficult, and many were effectively isolated.[6]

In the initial months of the Underground War, the Traitors committed many atrocities. The Graven Star Chapter, led by Captain Hol Beloth, overran Uranik arcology where it ritually sacrificed all captured defenders. The shelter of CV427/ Praxor and its 300,000 inhabitants was completely destroyed when cultists infiltrated it and detonated atomic warheads.[7b]

Dark Apostle Maloq Kartho sacrificed his own comrades, Foedral Fell and the last remnants of his forces, as part of a trap to lure Remus Ventanus out of one of the main arcologies,[8a] so Kartho could detonate a virus bomb inside it.[8b] Ventanus was able to stop the detonation and slay a daemonspawn-turned Hol Beloth. Kartho and the virus bomb were thrown into a warp rift that he himself had opened, averting Kartho's attempted genocide of the remaining Ultramarines and Loyalist survivors.[8c] However, Katho did not die, as he was transformed into Daemon Prince for his deeds.[9]

Slaughter at the Auroran arcology

A few months after the Underground War's start, Nurgle-corrupted Word Bearers attacked the Ultramarines' 547th Company at the Auroran arcology. After heavy fighting, the Traitors were able to corrupt Consul-Chaplain Ankarion, before massacring all remaining defenders and local civilians. However, a garbled transmission was sent before the Auroran arcology fell, alerting the 4th Destroyer Company of the Ultramarines' 22nd Chapter based at subterran bunkers at the Aesperia Praefecture. Led by Captain Valius Thesian, the unit set out on a rescue mission. Upon entering the arcology, the 4th Destroyer Company initially found only corpses of regular humans before being ambushed by mutated and possessed Word Bearers. The fighting soon devolved into close quarters-combat, and the Nemesis Destroyers quickly realized that the animated corpses of the 547th Company fought alongside the Traitors.[7c]

Despite taking heavy losses, the 4th Destroyer Company fought its way through the Traitors to the former Legion command strategium in the Auroran arcology. There, they found the corrupted and mutated Consul-Chaplain, accompanied by a retinue of living dead and Possessed. Furious at realizing Ankarion's betrayal, the Ultramarines charged. Ultimately, Captain Thesian sacrificed himself to kill Ankarion, allowing the 4th Destroyer Company to turn the tide of battle and defeat the remaining Traitor forces. Veteran Sergeant Cassian took command of the 4th Destroyer Company's survivors and they would report what had occurred in the arcology. This later led the Ultramarines' central command to destroy the Auroran arcology with atomic bombs.[7c]

Later fighting

Eventually, the Ultramarines sent some relief forces to support the Loyalist holdouts on Calth. In 009.M31, a contingent of Word Bearers led by Dark Apostle Kurtha Sedd overran an Ultramarine stronghold and captured Captain Vultius, attempting to use the latter as a vessel for Daemonic possession. This plan was foiled by Sergeant Aeonid Thiel who freed Vultius before the Ultramarines killed Sedd.[6]

A few Word Bearers would question their situation, realizing that their own Primarch had left them to die. One of these, a legionary later known as the "Anchorite", came to the conclusion that Lorgar's pact with Chaos had been a mistake and eventually surrendered to the Ultramarines.[4]

Eventually, the Ultramarines succeeded in purging all Traitors from Calth. The Ultramarines' 22nd Chapter proved to be instrumental in this victory, though the Chapter would never fully heal from the mental scars inflicted by the Battle of Calth and Underground War.[7c] Despite their great contributions to the survival of Calth's defenders, none of the Loyalist Mechanicum Taghmata were known to have survived the Underground War.[3d]


Though the forces of Chaos were purged from Calth and the planet eventually became an industrial powerhouse once more, it continued to suffer from the effects of the Horus Heresy into M42.[10]



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