Underworld War

The Underworld War
Date	007-014.M31
Scale	Planetary
Theatre	Shadow Crusade
Status	Ultramarines victory
Belligerents
Word Bearers	Ultramarines
Commanders and Leaders
Dark Apostle Maloq Kartho, Captain Hol Beloth, Dark Apostle Kurtha Sedd, Captain Foedral Fell, Consul-Chaplain Ankarion	Captain Remus Ventanus, Captain Steloc Aethon, Captain Vultius, Captain Valius Thesian, Sergeant Aeonid Thiel, Archmagos Cronn Barbarel
Strength
~30,000 Legionaries with support from Chaos cults and Daemons	~40,000 Legionaries with Imperial Army and Mechanicum support
Losses
Almost total	Heavy
Outcome
Destruction of remaining Word Bearer forces on Calth. Multiple arcologies lost or destroyed. The Anchorite surrenders to Ultramarine forces and is imprisoned.

The Underworld War was the postscript to the Battle of Calth. After both loyalist and traitor forces were forced to retreat into the planet's underground arcologies, both sides promptly started fighting over control of the irradiated shithouse that Calth had become. This conflict would last for the rest of the Heresy.

Preliminaries

By the final stages of the Battle of Calth, the planet was suffering from the aftereffects of the Word Bearers' bombardment and the radiation flares emanating from the poisoned star of Veridia. Recognizing that the planet's surface was shortly going to become uninhabitable, Captain Remus Ventanus of the 4th Company issued a general order to all remaining loyalists on the planet: they were to move immediately to the nearest arcology or cave system to take shelter until the radiation storms dissipated or the Ultramarines fleet was able to return and extract them. There were approximately forty thousand Ultramarines still alive on Calth, all of whom obeyed this order, along with the surviving loyalist elements of the Imperial Army and Mechanicum and as many civilian survivors as could be rounded up. The Word Bearers and Chaos forces still alive on the surface had the same idea, and soon both sides were fighting over the available caves and arcologies.

Major Engagements

The Word Bearers overran and captured several of the arcologies almost immediately, fortifying them against any counterattacks. Other groups of Word Bearers instead opted to prowl the tunnels and caves and wage guerrilla war against the Ultramarines. Others summoned daemons to bedevil and harass the Ultramarines, though the daemons were just as happy to kill the Word Bearers if they couldn't find anything else to get their teeth into. The Uranik arcology fell to Hol Beloth and the Graven Star chapter, who ritually sacrificed all the captured defenders to the Ruinous Powers. The deep shelter CV427/Praxor and two other arcologies were wiped out when Chaos cultists smuggled in some nukes and set them off, killing two million people in a single night.

One of the first major battles of the Underworld War took place in the Auroran arcology a few months after the beginning of the war. A group of Nurglite Word Bearers attacked the arcology and its defenders, the 547th Company of the Ultramarines. During the fighting, the Word Bearers were somehow able to corrupt Consul-Chaplain Ankarion, commanding officer of the defenders. Ankarion promptly turned on his former comrades and led the Word Bearers in massacring them and the two hundred thousand civilians who had taken shelter in the arcology. After this, the traitor forces built a shrine to Nurgle in the heart of the arcology, allowing his corruption to spread freely. When the 4th Destroyer Company of the Nemesis Chapter showed up to investigate the fragmentary distress call the 547th had sent, they found themselves fighting the Word Bearers and hordes of zombified Smurfs. The Destroyers blasted their way through to the center of the arcology, where they saw Ankarion standing with the Word Bearers, already wearing crimson armor and with his helmet turning into a mutated mass of eyes, mouths, and horns. Enraged, the Ultramarines charged their fallen battle-brother and his new allies. Captain Valius Thesian dueled Ankarion himself, only for the corrupted Chaplain to skewer him on an obsidian spear. Thesian promptly dragged himself down the spear, stabbed Ankarion in the neck, and triggered a melta bomb, killing them both. The Destroyers successfully purged the arcology, but their report would later lead the Legion command on the planet to sterilize the arcology with a nuke, since it was the only way to be sure.

Dark Apostle Maloq Kartho concocted a plan to ascend to daemonhood and kill Captain Ventanus at the same time. After stealing a virus bomb from the armories of CV427/Praxor, he ritually sacrificed Captain Foedral Fell and his troops to lure Ventanus and his Ultramarines out of their command center in Arcology X, which was one of the largest loyalist refuges on Calth and the nerve center of their resistance to the Word Bearers. Destroying it would cripple the Ultramarines and reap a great harvest of souls for the dark gods. Ventanus and his forces smashed Fell's fortifications, which were only lightly held by cult troops, and found Fell and his Word Bearers murdered within their own temple, their bodies transfixed with iron posts and arranged in a ritualistic pattern. Upon realizing that they'd been tricked, Ventanus ordered his men to evacuate, only for Fell and his Astartes to reanimate and attack them. As this was occurring, Kartho and Captain Hol Beloth, who was deteriorating into a daemonspawn, breached Arcology X and easily overran the defenders, who were mainly Imperial Army with only a few Ultramarines. Kartho armed the virus bomb and disabled its failsafes, then prepared to depart by using his athame to open a Warp rift. Ventanus arrived in the nick of time, having rushed back to the arcology in a Land Speeder. He killed Hol Beloth with his own athame, then used the Land Speeder to ram the virus bomb into the Warp rift, preventing it from detonating in the arcology. Impressively enough, Kartho neither died nor was turned into a That-Which-Shall-Not-Be-Named, but was rewarded for all the slaughtering he'd already done with ascension to daemonhood, becoming the daemon prince eternal Grey Knights/Ultramarines punching bag known as M'kar.

Outcome

The outcome of the war was never really in doubt, since the Word Bearers who'd been sent to Calth in the first place had been deemed expendable by Lorgar, meaning that they were never going to receive reinforcements or extraction. The Ultramarines, meanwhile, could rely on the promise Guilliman had made to send help as soon as he could. A relief force arrived around 009.M31, led by Sergeant Aeonid Thiel, and provided a much-needed shot in the arm to the loyalist forces. Thiel himself saved the life of Captain Steloc Aethon by preventing Dark Apostle Kurtha Sedd from sacrificing him to the Chaos gods, after which they killed Sedd. After seven long years of war, the last of the Word Bearers were sponged, purged, and blasted from the caverns and tunnels, but it wasn't much of a victory. Calth had still been trashed beyond repair and hundreds of thousands of civilians and thousands of Ultramarines had been killed, along with the entirety of the loyalist Mechanicum forces. Thousands more were mentally scarred or broken by the unremitting stress and horror of the battle, including the Nemesis Chapter, who had borne the brunt of some of the worst fighting.

That said, there proved to be a few silver linings. Calth proved to be salvageable; though its surface would never again support life, the arcologies had proven their worth, and over the succeeding millennia they would be expanded into grand underground cities that rivaled the grandeur of Macragge itself. Likewise, Aeonid Thiel acquired a great deal of practical wisdom on fighting Word Bearers and their daemonic allies, which would later be incorporated into Guilliman's Big Book of Space Marine War. Last of all, a few of the Word Bearers had come to realize that they had been abandoned by their primarch and legion and began to question the bargains that Lorgar had made. One of them actually surrendered to the Ultramarines and was later imprisoned on Terra. While confined, he returned to the worship of the Emperor and quietly helped promulgate the notion of his divinity through the Lectitio Divinitatus, making him an instrumental, if unsung, founder of the modern Imperial Cult.