Battle of Calth

The Battle of Calth
Date	007.M31
Scale	Intrasolar & planetary
Theatre	Shadow Crusade
Status	Pyrrhic Ultramarines victory
Belligerents
Word Bearers	Ultramarines
Commanders and Leaders
Kor Phaeron Erebus	Roboute Guilliman
Strength
~50,000 Legionaries with support from Imperial Army, Chaos cults, and Daemons	~200,000 Legionaries with Imperial Army support
Losses
~50,000 Legionaries, unknown number of Auxilia	119,422 Legionaries, half a million Auxilia
Outcome
Creation of the Ruinstorm, preventing passage out of the Five Hundred Worlds

"My brother, hear me. Warriors of the XVIIth Legion, hear me. This violence is against the code of the Legiones Astartes and against the will of our father, the Emperor. In the name of the Five Hundred Worlds of Ultramar, I implore you to cease fire and stand down. Open communication with me. Let us speak. Let us settle this. This action is an error of the most tragic kind. Cease fire. I, Roboute Guilliman, give you my solemn pledge that we will deal with each other frankly and fairly if these hostilities can be suspended. I urge you to respond."

– Roboute Guilliman, at the beginning of the battle

"Lorgar of Colchis. You may consider the following. One: I entirely withdraw my previous offer of solemn ceasefire. It is cancelled, and will not be made again, to you or to any other of your motherless bastards. Two: you are no longer any brother of mine. I will find you, I will kill you, and I will hurl your toxic corpse into hell’s mouth."

– Roboute Guilliman, shortly after

The Battle of Calth, also referred to as the Calth Atrocity, was one of the major battles of the Horus Heresy and the reason for most of the current hatred between the Word Bearers and Ultramarines.

Prior to the battle

After Big E. chastised and publicly shamed the Word Bearers by making them kneel in front of him and the Ultramarines, Lorgar was slightly miffed, fell to Chaos, started the Horus Heresy... you know the drill.

Horus ordered the Ultramarines to muster at Calth, ostensibly for a joint operation with the Word Bearers against some Orks. Because of his distance from the galactic center and the general absence of reliable communications in the Imperium, Guilliman had no idea that the Drop Site Massacre had happened or any inkling that anything was wrong at all. From his point of view there was nothing to suspect and he hoped that this action would mend his relationship with his brother, so he assembled (almost) all of his Legion at Calth and waited for Lorgar's troops to arrive.

On Lorgar's part, he and his brother Angron got together in the aftermath of the battles at Isstvan, collected three hundred thousand legionaries between them and double-timed it to the Ultramar sector, initiating the Shadow Crusade on the way and burning a "bloody trail across the eastern segmentum". The Ultramarines were ignorant of these early battles thanks to both the speed of the traitors' advance and the fact that they seldom left any survivors.

When preparing for the muster at Calth, Lorgar split his Legion into multiple groups. He kept the larger part for himself, but deployed 50,000 Marines chosen from the most unstable and least zealous elements of his legion to represent him in the hopes that they would be killed off. He also gave them the bulk of his fleet to maintain the ruse that the entire legion had arrived at the muster, when in fact they were already spread throughout the 500 Worlds causing generalized mayhem.

While Lorgar desired to get back at Guilliman, he had been informed through the counsel of Kairos Fateweaver (and it should be noted that this is still the only time both of its heads were compelled to speak the truth) that the upcoming battle would have been his opportunity to kill Guilliman if he wished, but if he did so it would actually hinder the traitors' efforts to reach Terra and potentially cause Horus to fail in the long run. We now know with hindsight that if Guilliman had died then the Imperium Secundus would not have happened and the resources and manpower that he gathered to support his backup Imperium would have been spread elsewhere. Therefore while Lorgar was present at the muster at Calth, he departed to continue the Shadow Crusade as soon as the battle started, leaving overall command to Kor Phaeron.

Opening

The Word Bearers were not terribly subtle about their shady intentions, but Guilliman and his Legion were unprepared for a treacherous attack from a presumed friendly force. The first action Erebus's forces took was to attack a maintenance vessel, the Campanile, and take it over without the Ultramarines noticing. The Word Bearers began a ritual that would implant Chaos-infected scrapcode into the Calth orbital defense network while they deployed to the planet's surface under the cover of beginning coordination with the Ultramarines.

Being the efficient pragmatists that they were, the Ultramarines had a system for post-battle analysis. One of the key components of this was the "Mark" system, where all forces would record a certain time for the official start of the battle (Mark Zero) and record actions based on that initial time point. Twenty minutes prior to Mark Zero of the Battle of Calth, the Campanile, moving at a significant percentage of lightspeed, crashed into the Calth orbital docks. Tens of thousands died in the resulting cascade of explosions, and dozens of ships were destroyed or crippled. Seconds later, the Word Bearers fleet opened fire on the Ultramarines. After Guilliman's urgent plea for a ceasefire was ignored and Lorgar openly declared his traitorous intentions, he gave the order to fire back, officially beginning the battle.

The battle

This is what you get for not double-checking that the Primarch's dead.

As the orbital defense network was helpless and the Ultramarines fleet was fighting for survival, the Word Bearers vessels could bombard Calth with impunity, which they did. After Guilliman put two and two together and realized this was a deliberately planned attack, he angrily phoned up Lorgar, promising to kill him and every one of his sons. Lorgar, laughing his ass off, summoned a swarm of daemons onto the Macragge's Honour which tossed Guilliman (who wasn't wearing a helmet) and his bridge crew into the void. The casualties included nearly all of the Ultramarines' senior commanders and the entire bridge staff.

On the ground, tens of thousands of Ultramarines and Imperial Army soldiers were cut down before they realized what was happening. The Word Bearers and their auxiliary units swept across the planet, exterminating everyone in their path, whether civilian or military. The loyalists who weren't killed in the initial attack struggled to rally in the face of this tremendous backstab. Their efforts were rendered nearly impossible by the loss of the planetary vox network and Mechanicum noosphere, which cut them off from the fleet and each other. Just eleven hours into the battle, the Ultramarine commanders estimated that over 100,000 legionaries were dead, leaving them severely outnumbered by the Word Bearers.

Nevertheless, several large battle groups led by a mix of Ultramarines, Army officers, and skitarii were able to begin fighting back against the Word Bearers. Their efforts eventually centered around the governor's palace at Leptius Numinus, where Mechanicum magos Meer Edv Tawren was able to activate an antique data-engine that let them reestablish vox communications. They also discovered the existence of a killcode created by Calth's ranking Mechanicum server before his death in the initial attack that would allow them to clear the scrapcode from the noosphere and retake control of the orbital weapons grid. Led by Captains Remus Ventanus and Lyros Sydance, the Ultramarines stormed a civilian spaceport in the city of Lanshear that possessed a data-engine capable of operating the weapons grid, while a small group remained at Leptius Numinus to coordinate the loyalist forces for as long as possible. The Word Bearers, having noticed what was going on, eventually overran Leptius Numinus and counterattacked Lanshear in overwhelming numbers. The Ultramarines were on the verge of being destroyed when reinforcements showed up from all over the planet, vectored there by a junior magos who had remained behind at Leptius Numinus. The loyalists held the Word Bearers back long enough for Tawren to successfully deploy the killcode and seize control of the weapons grid. She immediately demolished the Word Bearers' forces on the ground and in orbit.

Whilst all this was happening, Aeonid Thiel, an Ultramarines sergeant who was under censure for planning and simulating Astartes vs. Astartes combat situations, went into action. He assembled a group of Ultramarines, Army troops, and lower-decks crew and armed them with weapons from Guilliman's personal collection, then led them to retake Macragge's Honour from the daemonic forces that Lorgar had summoned. They managed to clear most of the ship after about ten hours of vicious fighting, picking up more Space Marines and human crew along the way. Thiel eventually found Guilliman on the outside of the ship, still not wearing a helmet, killing Word Bearer boarding parties with his bare hands.

While the Ultramarines were still in a bad spot, they had rallied and were fighting back with a ferocity that startled the Word Bearers. Kor Phaeron boarded the Zetsun Verid shipyard and used it to reactivate some of the orbital defense platforms. After trashing everything else he could hit in the Veridian system, Phaeron ordered the grid to begin firing into Calth's sun. Magos Tawren was able to stop him with the killcode, but not before lethal damage had been inflicted on the star, corrupting it so it would produce massive amounts of radiation.

Guilliman led an attack on Zetsun Verid, where he and Kor Phaeron confronted each other. The latter managed to wound the Ultramarines primarch with his athame and tried to corrupt him as Erebus had done to Horus, but Guilliman told him to fuck off with that Chaos nonsense and ripped one of his hearts out. The Word Bearer, being a little bitch, teleported away with his main forces, allowing the Ultramarines to retake the platform and purge the scrapcode. Guilliman ordered the Macragge's Honour to pursue the Word Bearers' flagship, the Infidus Imperator. The latter was destroyed after a vicious void battle.

With the Word Bearers' fleet smashed, the orbital weapons grid back in loyalist hands, and lethal levels of radiation now bombarding Calth's surface, Guilliman ordered the evacuation of as many civilians and military personnel as possible. Those who could not be evacuated in time retreated into Calth's underground arcologies. The surviving Word Bearers on the surface had the same idea, and soon the two sides were engaging in a brutal series of close-quarters battles in the Calth tunnel networks. This would become known as the Underworld War.

Aftermath

Though technically an Ultramarines victory due to the fact the attacking Word Bearers were exterminated, Lorgar was happy with the result, seeing it as a sacrifice to the Dark Gods (causing the Ruinstorm), a means of purging his Legion of those whose devotion to Chaos was suspect, and as a way of irrevocably pissing off Guilliman.

And piss him off it did. The Mark of Calth is still running and will continue to run until the Ultras finally hunt down and kill the last Word Bearer. So the timer that started when Guilliman ordered the Ultramarines to return fire continues to run, hour by hour, even 10,000 years later, through every warp storm, rebellion, and incursion the XIII have faced, recording every action they have taken against any Word Bearer for 100 centuries. It is likely the most comprehensive list of casualties amongst any record in the galaxy.

The good thing that came out of this Battle is that the Ultrasmurfs got their butts handed to them, so if you sadistically enjoy the deaths of hundreds of thousands of smurfs, then you will love this battle.