Event · M31
Underworld War
The Underworld War, also recorded as the Underground War, was the underground phase of fighting on Calth from 007 to 014.M31. It was fought in the Veridia system of Ultramar between the surviving Ultramarines garrison and Word Bearers forces left stranded there after the main Battle of Calth. The war followed directly from the Battle of Calth, in which the Word Bearers, acting for the Warmaster Horus, struck the Ultramarines without warning. Unable to destroy the Legion outright, the traitors corrupted Calth’s sun, Veridia, with warp sorcery; Roboute Guilliman’s forces stopped it going nova, but the star was left poisoned, irradiating the surface. Lorgar withdrew most of his fleet, deliberately abandoning the Word Bearers companies he judged least reliable, while Guilliman evacuated what forces he could and ordered the remainder, together with the Ultramar Auxilia, Calth Irregulars, and the loyalist Mechanicum under Archmagos Cronn Barbarel, into Calth’s vast subterranean arcologies, self-contained cavern cities. Captain Remus Ventanus of the 22nd Chapter led the retreat underground and became the effective leader of the besieged loyalists, one of the senior Ultramarines commanders who had directed the Legion’s wider defence of Calth, alongside First Master Marius Gage. Cut off from the wider Heresy, the Word Bearers fortified several arcologies while other companies raided through the tunnels beneath. The Graven Star Chapter, under Captain Hol Beloth, overran the Uranik arcology and slaughtered its defenders, and cultists destroyed the Praxor shelter with atomic devices, killing some 300,000 civilians. Dark Apostle Maloq Kartho sacrificed his ally Foedral Fell to lure Ventanus into a trap built around a virus bomb; Ventanus foiled the attack, killed the daemonically transformed Beloth, and cast Kartho into a warp rift, though Kartho survived to be reborn as a Daemon Prince. Nurgle-tainted Word Bearers later overwhelmed the Ultramarines’ 547th Company at the Auroran arcology and corrupted its Consul-Chaplain, Ankarion; Captain Valius Thesian of the 4th Destroyer Company fought through to the arcology’s command post and sacrificed himself to kill the fallen chaplain. In 009.M31 the Dark Apostle Kurtha Sedd captured Captain Vultius for daemonic possession, until Sergeant Aeonid Thiel freed him and killed Sedd.
See where Underworld War occurred on the galactic map during the Horus Heresy.