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
Entirety of the Legion with Imperial Army support	~200,000 Legionaries with Imperial Army support
Losses
~50,000 Legionaries, unknown number of Auxilia	119,422 Legionaries, half a million Auxilia
Outcome
Created the Ruinstorm, preventing passage out of the Five Hundred Worlds

The Battle of Calth 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.

One of Lorgar's desires was to get back at Guilliman, and in the opening stages of the Heresy he would get his chance. Horus ordered the Ultramarines to muster at Calth, ostensibly for a joint operation with the Word Bearers against Orks. Guilliman, who at that point suspected nothing and hoped that this action would mend his relation with his brother, assembled (almost all of) his Legion and waited for Lorgar's troops to arrive.

Opening

The Word Bearers were not terribly subtle about their shady intentions, but Guilliman and his Legion were both 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' forces began a ritual that would implant Chaos-infected scrapcode into the Calth orbital defense network, while they deployed forces to the planets 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. Thousands died in the explosion, and seconds later the Word Bearers fleet opened fire on the Ultramarines. After Guilliman's urgent plea for a ceasefire was ignored, he gave the order to fire back, officially beginning the battle.

The battle

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 entire bridge crew into the void.

Aeonid Thiel, an Ultramarines sergeant who had been punished for planning and simulating Astartes vs. Astartes combat situations, led the attack to retake the Macragge's Honour from the daemonic forces. They managed to do so after about ten hours of vicious fighting. 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. Thiel and co. found Guilliman on the outside of the Macragge's Honour, still not wearing a helmet, killing Word Bearer boarding parties with his bare hands.

While the Ultramarines were still in a bad spot, they were beginning to fight back. Kor Phaeron proceeded to reactivate some of the orbital defense platforms and began firing into Calth's sun. With fatal radiation bombarding Calth's surface, both sides' forces on the planet retreated underground, engaging in a brutal series of close-quarters battles in the Calth tunnel networks.

Guilliman led an attack on the platforms, where he and Kor Phaeron confronted each other. The latter tried to stab the Ultramarine primarch with an athame, 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 Bearer's flagship, the Infidus Imperator. The latter was destroyed after a vicious void battle.

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) 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.