View source for Battle of Calth

{{ambig|subject=historical battle|disambiguation=Betrayal at Calth}}
{{QuoteBox|quote = '''''"Victory is a fickle mistress, failure a haphazard executioner; for those with the courage to preserve will not abide their dictates."'''
|source = - Attributed to [[Tetrarch of Ultramar|Tetrarch]] [[Eikos Lamiad]], from the dedication of the "[[Defence of Bathor]]" monument, [[Calth Holophusikon]]{{Fn|9f}}}}
{{Battle
|Name         =The Battle of Calth
|Image        =Guilliman space battle.jpg
|Image caption=[[Roboute Guilliman]] takes the battle to the void above Calth{{Fn|2ae}}
|Conflict     =[[Horus Heresy]]{{Fn|2a}}
|Date         =007.[[M31]]; [[Underground War]] continues until 014.[[M31]]{{Fn|11}}
|Location     =[[Calth]], [[Veridian System]], [[Ultramar]]{{Fn|2a}}
|Outcome      =Pyrrhic Loyalist Victory{{Fn|2ad}}<br>-Traitor forces destroyed{{Fn|2ad}}<br>[[Ruinstorm]] created{{Fn|2g}}
|Combatant1   =[[File:Chaos.png|30px]] [[Chaos Space Marines|Traitors]]{{Fn|9a}}
|Combatant2   =[[File:Aquila1transparent.png|30px]] [[Imperium]]{{Fn|9a}}
|Commanders1  =[[File:WB Icon.png|20px]] [[First Captain]] [[Kor Phaeron]] (WIA){{Fn|2j}}<br>[[File:WB Icon.png|20px]] [[Force Commander]] [[Foedrall Fell]] (KIA){{Fn|2w}}<br>[[File:WB Icon.png|20px]] [[First Chaplain]] [[Erebus]]{{Fn|2u}}<br>[[File:WB Icon.png|20px]] [[Force Commander]] [[Hol Beloth]] (KIA){{Fn|2w}}<br>[[File:WB Icon.png|20px]] [[Force Commander]] [[Morpal Cxir]] (KIA){{Fn|2n}}<br>[[File:WB Icon.png|20px]] [[Dark Apostle]] [[Maloq Kartho]]{{Fn|6a}}<br>[[File:WB Icon.png|20px]] [[Space Marine Captain|Captain]] [[Sorot Tchure]] (WIA){{Fn|2f}}<br>[[File:WB Icon.png|20px]] [[Dark Apostle]] [[Kurtha Sedd]] (KIA){{Fn|12}}<br>[[File:WB Icon.png|20px]] [[Dark Apostle]] [[Zardu Layak]]{{Fn|9d}}
|Commanders2  =[[File:UL Icon.png|20px]] [[Primarch]] [[Roboute Guilliman]]{{Fn|10}}<br>[[File:UL Icon.png|20px]] [[Tetrarch of Ultramar|Tetrarch]] [[Eikos Lamiad]]{{Fn|2z}}<br>[[File:UL Icon.png|20px]] [[Tetrarch of Ultramar|Tetrarch]] [[Tauro Nicodemus]]{{Fn|2z}}<br>[[File:UL Icon.png|20px]] [[Chapter Master|First Master]] [[Marius Gage]]{{Fn|2n}}<br>[[File:UL Icon.png|20px]] [[Space Marine Captain|Captain]] [[Remus Ventanus]]{{Fn|2f}}<br>[[File:UL Icon.png|20px]] [[Chapter Master]] [[Klord Empion]]{{Fn|2}}<br>[[File:UL Icon.png|20px]] [[Space Marine Captain|Captain]] [[Saur Damocles]]{{Fn|2c}}<br>[[File:UL Icon.png|20px]] [[Space Marine Captain|Captain]] [[Steloc Aethon]]{{Fn|2}}<br>[[File:UL Icon.png|20px]] [[Space Marine Captain|Captain]] [[Erikon Gaius]]{{Fn|2}}<br>[[File:UL Icon.png|20px]] [[Space Marine Captain|Captain]] [[Teus Sullus]]{{Fn|2}}<br>[[File:UL Icon.png|20px]] [[Space Marine Captain|Captain]] [[Lyros Sydance]]{{Fn|2}}<br>[[File:UL Icon.png|20px]] [[Space Marine Captain|Captain]] [[Aurian (Captain)|Aurian]]{{Fn|13}}<br>[[File:UL Icon.png|20px]] [[Captain]] [[Honorius Luciel]] (KIA){{Fn|2a}}<br>[[File:UL Icon.png|20px]] [[Space Marine Sergeant|Sergeant]] [[Aeonid Thiel]]{{Fn|2}}<br>[[File:UL Icon.png|20px]] [[Space Marine Sergeant|Sergeant]] [[Anchise]]{{Fn|2c}}<br>[[File:Machina Opus.png|20px]] [[Master of Skitarii|Master]] [[Arook Serotid]] (KIA){{Fn|2}}<br>[[File:Imperial Army Symbol.jpg|20px]] [[Colonel]] [[Sparzi]]{{Fn|2}}<br>[[File:Machina Opus.png|20px]] [[Magos]] [[Cronn Barbarel]]{{Fn|9c}}
|Strength1    =[[File:WB Icon.png|20px]] More than 50,000 [[Word Bearers]]<br>[[File:Daemons Icon.png|20px]] Countless [[Daemons]]{{Fn|9a}}<br>[[File:Chaos.png|20px]] ~500,000 [[Cultists]]{{Fn|9a}}<br>[[File:Suturvora Icon.png|20px]] [[Legio Suturvora]], ~130 [[Titan]]s{{Fn|9a}}<br>[[File:WB Icon.png|20px]] Bulk of Word Bearers fleet{{Fn|9a}}
|Strength2    =[[File:UL Icon.png|20px]] 185,923 [[Ultramarines]]{{Fn|9a}}<br>[[File:Imperial Army Symbol.jpg|20px]] >~1 million [[Ultramar Auxilia|Auxilia]] and [[Imperial Army|Army]]{{Fn|9a}}<br>[[File:Machina Opus.png|20px]] [[Skitarii]] Cohorts{{Fn|9a}}<br>[[File:Titan Icon.png|20px]] ~118 [[Titans]]{{Fn|9a}}<br>[[File:UL Icon.png|20px]] Bulk of Ultramarines fleet{{Fn|9a}}
|Casualties1  =20,000 Astartes killed in the initial battle, remainder persecute the underground war over the next decade{{Fn|9b}}<br> More than 65 titans destroyed{{Fn|9b}}<br>All cultists killed{{Fn|9b}}<br>Half of Word Bearer fleet destroyed{{Fn|9b}}<br>Flagships ''[[Infidus Imperator]]'' and ''[[Destiny's Hand]]'' destroyed{{Fn|2z}}
|Casualties2  = 119,422 Astartes killed{{Fn|9b}}<br>28,392 Astartes crippled{{Fn|9b}}<br>100 Titans destroyed{{Fn|9b}}<br>4/5th of fleet destroyed{{Fn|9b}}<br>At least 500,000 Army & Mechanicum casualties{{Fn|9b}}
}}
The '''Battle of Calth''' was a major battle of the [[Horus Heresy]], in which the [[Word Bearers]] [[Traitor Legion]] attempted to exterminate the [[Ultramarines]] [[Space Marine Legion|Legion]], on the orders of [[Warmaster]] [[Horus]], who anticipated that [[Guilliman]], the Ultramarines' [[Primarch]], would not join his rebellion against the [[Emperor]].{{Fn|4x}}

===Early Events===
During the [[Great Crusade]], approximately forty years before the Heresy, the [[Emperor of Mankind]] rebuked the Word Bearers and their [[primarch]], [[Lorgar|Lorgar Aurelian]], for fostering idolatrous worship of the Emperor, and for violent atrocities against those who were slow to adopt their beliefs.{{Fn|5d}} As an object lesson, the Emperor ordered Guilliman and the Ultramarines to raze the city of [[Monarchia]], the Word Bearers' proudest achievement, to the ground. He then forced Lorgar and his entire Legion to kneel in the city's ashes before him, Guilliman, and [[Malcador the Sigillite]]. These events sent Lorgar down a dark path that would eventually lead him to swear allegiance to [[Warmaster]] [[Horus|Horus Lupercal]] and the [[Gods of Chaos]], and set the entire Word Bearers Legion in a vendetta against the Ultramarines.{{Fn|4x}} While Lorgar undoubtedly was motivated against the Ultramarines on a personal level, he had earlier been told by [[Fateweaver]] that killing Guilliman would ultimately doom the Traitors cause.{{Fn|10}}

After the massacres of loyalist forces at [[Isstvan III]] and [[Isstvan V|V]], and having gained the allegiance of Lorgar and the other Traitor Primarchs, Horus believed that the Ultramarines - the largest of all the Space Marine Legions - were the last and only significant obstacle to his total victory over the Emperor.{{Fn|2k}}

==The Battle==
===The Setting===
Horus ordered Guilliman to gather the majority of his Legion at [[Calth]], one of the many planets in the [[Ultramar]] Coalition. The ostensible reason for this order was an anticipated incursion by [[Ork]]s in the [[Veridian System]].{{Fn|5a}} As if the muster of the Ultramarines was not enough, Horus ordered that the [[Word Bearers]] would go into battle alongside the Ultramarines.{{Fn|2c}}

Some of the more cynical Ultramarines, including Guilliman himself, guessed that the operation was more a political exercise than a real war: Horus, the newly-appointed [[Warmaster]], wanted to demonstrate his authority by issuing an order to the largest of all the [[Space Marine Legion]]s, and give the recently-disfavored Word Bearers a chance to gain some reflected glory by fighting alongside the Ultramarines.{{Fn|2c}}

Guilliman was ignorant of Horus' treachery, and obeyed his brother's order without question, mustering twenty of the Legion's twenty-two [[chapters]] at Calth.{{Fn|2c}}{{Fn|5a}}

===Opening Moves===
Later analyses of the Battle would criticize that there were plenty of signs warning of the Word Bearers' intentions, but the Ultramarines, for all their martial prowess, failed to notice them. The Ultramarines were simply unprepared for the novelty of fighting against a brother [[Space Marine Legion|Legion]], especially one that based its strategy on manipulating the forces of the [[Empyrean]]. Lorgar took full advantage of this blindness.{{Fn|2c}}

Approximately one hundred thirty-six hours before '''Mark Zero''', (the official start of the Battle, by the Ultramarines' reckoning){{Fn|2b}}, the Word Bearers boarded the fleet tender ''[[Campanile]]'' and massacred its crew in the outlying reaches of the Veridian system.{{Fn|2a}}

The Word Bearers began to arrive in-system at around '''Mark -1.24.00''', docking their fleet with Calth's orbital shipyards and deploying ground forces – including Legionnaires, human auxiliaries, and [[Titan]]s – to the surface of Calth, presumably to await embarkation alongside the Ultramarines.{{Fn|2a}}

Sixty hours before '''Mark Zero''', the Word Bearers and their [[cultist]] soldiers commenced the first of their [[Chaos]] rituals, which would cause havoc across the planet. The Ultramarines picked up the sounds of chanting over the [[vox]], but dismissed them as solar flare distortion. Likewise, nearly all of the Ultramarines' lapsed [[psyker]]s, who had been redeployed to ordinary [[Tactical Squad|tactical]] places after the Emperor's Decree at the [[Council of Nikaea]], began experiencing headaches. Since the Emperor had forbidden the use of their psychic abilities, they dismissed the sensations as fatigue, and did not mention them to their superiors, or even to each other.{{Fn|2c}}

At around '''Mark -16.44.00''', [[Magos]] [[Uhl Kehal Hesst]] noticed a foreign [[scrapcode]] infesting the [[cogitator]]s controlling Calth's orbital defence network. The source of this scrapcode was a [[Chaos]] ritual being conducted in secret at the camp of the Word Bearers' auxiliaries, [[The Brotherhood of the Knife]].{{Fn|2d}} This ritually-produced scrapcode, known as the [[Octed]], was finally implanted at '''Mark -7.55.09'''.{{Fn|2e}}

Approximately one hour before '''Mark Zero''', [[First Chaplain]] [[Erebus]] arrived by [[teleport]] on the Satric Plateau, north of [[Numinus City]], to commence a dark ritual.{{Fn|2f}}

===Assault===
Twenty minutes before '''Mark Zero''', the ''Campanile'', which had bypassed Calth's outer defence grid, accelerated to full sub-light velocity. At the same time, [[Sorot Tchure]], a [[Space Marine Captain|Captain]] of the Word Bearers who had boarded the Ultramarines' cruiser ''[[Samothrace]]'', opened fire on Captain [[Honorius Luciel]] and his entourage, killing them outright.{{Fn|2f}} Luciel and his seventeen men were regarded by many as the first "official" casualties of the Battle of Calth.{{Fn|2a}}

The ''Campanile'' streaked into Calth's primary shipyard like an enormous missile, annihilating numerous warships and killing thousands in less than two seconds. The impact unleashed a massive electromagnetic pulse that disabled the majority of the remaining ships and, in conjunction with the scrapcode attack, also disabled Calth's weapons grid.{{Fn|2g}} Ten seconds after the ''Campanile'''s murderous journey, the Word Bearers' fleet opened fire on the Ultramarines' disabled ships, destroying them at will.{{Fn|2h}} Enormous pieces of the destroyed ships and space stations fell to Calth's surface, causing devastating explosions and tsunamis.{{Fn|2h}} Believing they were under attack, the Ultramarines and their Army auxiliaries naturally sought to link up with their counterparts from the Word Bearers, and were massacred as their once-brothers opened fire on them. The Ultramarines of the 111th and 112th Companies were among the first to be slaughtered.{{Fn|2i}}

From the bridge of the disabled flagship ''[[Macragge's Honour]]'', Guilliman swiftly deduced that the Word Bearers were opening fire on them, but attributed it to mistake on Lorgar's part. He transmitted urgent pleas for cease-fire, but when Lorgar refused to stop after a direct lithocast communication, Guilliman reluctantly authorized his forces to defend themselves, officially commencing the Battle of Calth at '''Mark Zero'''.{{Fn|2i}}

===Prosecution===
Tchure and his elite warriors slaughtered the remainder of the ''Samothrace'' crew, and, at their signal, [[Kor Phaeron]] teleported onto the ship, which they then docked with Calth's primary orbital defence platform. The Word Bearers' fleet, unchallenged, moved into low orbit above Calth's southern hemisphere, and began bombarding it, causing millions of casualties, both Ultramarine and civilian alike.{{Fn|2j}}

At approximately '''Mark 1.38.00''', Guilliman, who was sifting innumerable pieces of data in an attempt to understand the reasons for the Word Bearers' actions, received notice of the ''Campanile'' boarding. Concluding that Lorgar had planned the whole attack, Guilliman rescinded his appeals for cease-fire and instead transmitted his vow to kill his former brother and his entire Legion. Where Guilliman's prior hails had been ignored, it amused Lorgar to respond to this one, explaining with relish that his Legion and [[Chaos Space Marine|others]] had joined the banner of [[Warmaster]] [[Horus|Horus Lupercal]] in rebellion against the Emperor, [[Battle of Isstvan III|already]] [[Drop Site Massacre|massacred]] [[Loyalist]] forces at [[Isstvan III]] and [[Isstvan V|V]], and, with the Ultramarines destroyed, there would be no force left that could stop Horus from overthrowing the Emperor. Guilliman barely had time to absorb this shock before Lorgar spoke an incantation, summoning a swarm of [[daemon]]s that smashed into the ''Honour'' bridge, venting it to space.{{Fn|2k}} After this, Lorgar left command of the Calth operation to Kor Phaeron, and took the rest of the Word Bearers' force deeper into [[Ultramar]].{{Fn|7}} At '''Mark 4.55.00''', Kor Phaeron and his men had completed the taking over of Calth's weapons grid, and opened fire on the surrounding planets and planetoids in the Veridian system, beginning with the manufactoria asteroid Veridia Forge.{{Fn|2l}}

===Engagement===
[[Image:BrotherhoodKnife.jpg|thumb|right|200px|Word Bearers [[Cultist]]s battle the Ultramarines]]
The Word Bearers had achieved complete surprise, and managed to slaughter half or more of the loyalist muster before the latter could begin to react, but had reckoned without the intractable fighting spirit of the Ultramarines.

[[Space Marine Captain|Captain]] [[Remus Ventanus]] of the 4th Company had escaped the Word Bearers' surprise attack on the Northern Continent by the slimmest of margins, and, while escaping the pursuing Word Bearers, linked up with a group of [[Skitarii]] who had been guarding Numinus City's defence cogitators. By good fortune, they were equipped with back-up [[vox-caster]]s on a separate network, that allowed Ventanus to establish contact with another Captain of the 4th, [[Lyros Sydance]]. They coordinated a rendezvous at [[Leptius Numinus]], a former [[Governor]]'s palace on the [[Plains of Dera]].{{Fn|2l}}{{Fn|2m}}

Entrenching themselves at the palace, Ventanus and his men came under attack by the Word Bearers; when informed, Kor Phaeron at first considered it a diversion from the force's primary mission, but the ferocity of the Ultramarines' resistance led him to reconsider and re-task the force's commander, [[Morpal Cxir]], to exterminate them. Fortunately, the palace was equipped with a functioning data-engine, which Magos Hesst's deputy, [[Meer Edv Tawren]] was able to activate and use to re-establish [[auspex]]es and communications. The Word Bearers attacked in overwhelming numbers, but were held back, until they were ambushed from behind and defeated by the approach of Captain Sydance's force.{{Fn|2n}}{{Fn|2o}}{{Fn|2p}}{{Fn|2q}}

At '''Mark 11.06.00''', vox contact between the disparate units of Ultramarines all over the planet was finally restored.{{Fn|2r}}

Aboard the ''Macragge's Honour'', [[Chapter Master|First Master]] [[Marius Gage]] was maimed and nearly killed by the daemons overrunning the ship, but was saved by [[Space Marine Sergeant|Sergeant]] [[Aeonid Thiel]], leading an ad hoc party of Ultramarines to cleanse the ship. Thiel had been marked for censure for preparing theoretical combat scenarios that involved [[Space Marines]] fighting other Space Marines; this, ironically, made him most able to adapt to the bizarre tactical situation now confronting the Ultramarines. With Gage's approval, he led his party to eliminate the remaining daemons from the ship.{{Fn|2n}} At '''Mark 11:40.00''', Thiel and his men cleared the way to the ''Honour'' auxiliary bridge, allowing Gage to re-take control of the ship, just in time to communicate with Ventanus on the surface.{{Fn|2r}}
[[Image:HHCalth.jpg|thumb|right|200px|Ultramarines battle on ruined Calth]]
By the time Gage was able to see a near-complete picture, the odds were grim: half or more of the Ultramarines' muster had been destroyed (as many as one hundred thousand Legionnaires or more){{Fn|2m}}, the fleet had been reduced to a fifth of its original strength{{Fn|2t}}, and the survivors planetside had been reduced to roughly thirty thousand Legionnaires and two hundred thousand Army regulars and Mechanicum soldiers, split into roughly seventy disparate groups around the planet; in every respect, they were grossly outnumbered by the Word Bearers. As Magos Tawren reflected, any other force than the Ultramarines would have conceded defeat already.{{Fn|2s}}{{Fn|2v}}

The fleet's only saving grace was that the Word Bearers were refraining from destroying the largest capital ships, instead using boarding parties to capture them intact. Gage mobilised void-parties to repel these boarders, led by Thiel. It was during this desperate fight, when the Ultramarines were outnumbered eight to one, that Guilliman reappeared. He had been missing for hours after the destruction of the ''Honour'''s bridge, but his enhanced physiology allowed him to survive the near-vacuum outside the ship. Guilliman slew several Word Bearers with his bare hands before Thiel persuaded him to re-board the ship.{{Fn|2t}}

On the surface, havoc entered Leptius Numinus in the form of the [[daemon]] [[Samus]], who had "entered" the compound in the body of the captured Commander Cxir. Ventanus only managed to slay it by the slimmest of margins.{{Fn|2s}}{{Fn|2t}} In coordination with Erebus on the surface, who had nearly completed his ritual, Kor Phaeron turned the orbital platform's weapons onto the [[Veridia|Veridian sun]] itself.{{Fn|2u}}

===Endgame===
Magos Tawren discovered that, during the last moments of his life, Magos Hesst had formulated a code which would allow her to purge the Word Bearers' scrapcode. However, the data-engine at Leptius Numinus was not powerful enough to re-take control of Calth's defence grid. That task was left to the Ultramarines in orbit.{{Fn|2v}}
[[Image:KPGulliman.jpg|thumb|right|200px|Kor Phaeron attempts to corrupt Guilliman with his [[anathame]] dagger]]
Guilliman assembled a force of fifty Ultramarines to [[teleport]] aboard the platform. While they were assaulting the main control center, Kor Phaeron was able to stun Guilliman with powers granted by the [[Warp]]. While Guilliman was temporarily helpless, Phaeron bent over him with a [[athame|ritual knife]]. He could have killed the primarch, but was intoxicated with the idea of converting Guilliman to [[Chaos]], just as Erebus had [[anathame|converted Horus]]. This was a fatal mistake; although his flesh was pierced, Guilliman was resistant to the dark influence of Phaeron's blade, and tore Phaeron's [[primary heart]] from his chest with his massive [[Lightning Claw|armour claw]]. This was not enough to kill Phaeron, but was enough to drive him and his [[Gal Vorbak|elite guard]] from the platform. Sergeant Thiel swiftly destroyed the platform's data systems, breaking the Word Bearers' control of the defence grid.{{Fn|2w}}{{Fn|2y}}{{Fn|2z}}

While part of his force stayed behind to guard the vital data-engine, Ventanus led the majority to link up with an armoured element from the 4th Company to lead a counter-attack on the Word Bearers, led by [[Hol Beloth]] and [[Foedral Fell]].{{Fn|2w}}{{Fn|2x}}{{Fn|2y}} Grossly outnumbered, they received unexpected support from the remnants of the 111th and 112th Companies under Sergeant [[Anchise]], the 19th Company under Captain [[Steloc Aethon|Aethon]], [[Titan]]s under [[Tetrarch of Ultramar|Tetrarch]] [[Tauro Nicodemus]], and another relief force led by Tetrarch [[Eikos Lamiad]] and the [[Dreadnought]], [[Telemechrus]].{{Fn|2z}} Under normal circumstances, this would have merely delayed the Ultramarines' ultimate defeat, but the actions aboard the orbital platform allowed Magos Tawren to input the code and re-take control of the orbital weapons systems. She immediately opened fire on the Word Bearers on the ground, and their fleet in orbit, exacting a swift revenge on both. The ''Macragge's Honour'' likewise joined the fray, opening fire on the suddenly wrong-footed Word Bearer vessels.{{Fn|2z}}

Phaeron and the survivors of his party teleported to the Word Bearers' flagship ''[[Infidus Imperator]]'', which began to retreat out of system. Guilliman furiously ordered Gage to pursue in the ''Macragge's Honour'', leading to [[Battle of the Macragge's Honour|one of the most infamous naval duels in Imperial history]].{{Fn|2z}} Although the Ultramarines had prevented the Veridian sun from going nova, the star was irreversibly poisoned and changed, and began to bombard Calth with lethal radiation. Guilliman was forced to order as complete an evacuation of the system as was possible, in the short time they had left, while Ventanus led the surviving ground forces into Calth's gargantuan cavern system, knowing it would be months, or years, before the fleet could return to extract them.{{Fn|2z}}{{Fn|2aa}} At '''Mark 23:43.00''', the remnants of the Ultramarines' fleet translated out of system, signaling the end of that phase of the Battle of Calth, and, as the surviving Word Bearers followed the loyalist survivors into the caverns, began the phase known as the [[Underground War]].{{Fn|2aa}}{{Fn|2ac}}

On the Satric Plateau, Erebus had completed his dark ritual, and, believing the Ultramarines finished as a threat to Horus, cut a hole in the materium and escaped the planet's death with the host of daemons he had summoned.{{Fn|2z}} He was, however, followed in short order by [[Oll Persson]], a retired soldier of the [[Imperial Army]] who had, guided by [[John Grammaticus]], led a small force of human survivors to the Plateau. The outcome of his actions is as yet unknown.{{Fn|2ab}}

===A Secret Mission===
While the remnants of the Ultramarines and the Imperial Army were making their exodus to the caves, to escape the poisoning of Calth's atmosphere, the 21st Company, under Captain [[Erikon Gaius]], were dug in at one of the railway tunnels leading to Numinus City.{{Fn|5b}} In the midst of their defence from the attacking Word Bearers, [[Nathaniel Garro]], former [[Battle-Captain]] of the now-Traitor [[Death Guard]], arrived on a secret mission from [[Malcador the Sigillite]], to recruit [[Tylos Rubio]], a former [[Codicier]] of the Legion. At first, Rubio refused to abandon his brothers, but was left with no choice when, in the face of the Word Bearers' next, overwhelming assault, he unleashed his dormant [[psyker]] abilities, saving his company but making him an outcast for disobeying the [[Decree of Nikaea]]. He and Garro left the planet in Garro's [[Stormbird]].{{Fn|5c}}{{Fn|5d}}{{Fn|5e}}

===The Battle for Ithraca===
{{main|Betrayal at Ithraca}}
The city of '''Ithraca''' was the staging point for two [[Titan Legion]]s assigned to the Veridian undertaking, the [[Legio Praesagius]] (the "True Messengers") and the [[Fire Masters]]. As the Titans were boarding their ships, the Fire Masters turned on their allies and opened fire, at the same time that the Word Bearers fleet in orbit launched orbital bombardment.{{Fn|8a}} Many of the Messengers' Titans were destroyed in the surprise attack, but quick thinking by the remaining commanders pulled the survivors into the streets of Ithraca, forcing the Fire Masters to extend their line and pursue.{{Fn|8b}}

The center of the battle soon revolved around the wreck of the heavy transport ship ''Arutan'', which had crashed in the middle of the city after being struck by orbital fire. The ''Arutan'' had the majority of the True Messengers' Titans aboard, and loyalist forces fought to open her cargo doors and unleash them into the fight, while the traitor forces fought just as ferociously to prevent this.{{Fn|8b}}  The loyalist cordon protecting the ship was close to collapsing, when the doors were finally opened, bolstering their line and allowing them to counterattack the Fire Masters. The ultimate outcome of the battle is not known.{{Fn|8c}}

===The Destruction of the ''Furious Abyss''===
While Lorgar had concentrated his Legion's offensive on Calth, a separate assault on the Ultramarines' homeworld of [[Macragge]] had been prepared. A massive battleship known as the ''[[Furious Abyss]]'' had been secretly constructed on a [[Jupiter|Jovian]] moon called [[Thule (Moon)|Thule]], under the auspices of [[Kelbor-Hal]].{{Fn|3a}} It was equipped with [[cyclonic torpedo]]es and [[Virus bomb|viral bombs]], ready to kill the world and its moon, [[Formaska]].{{Fn|3b}} Fortunately, however, it was intercepted in space by an ad hoc force led by Ultramarines Captain [[Lysimachus Cestus]] and destroyed, though at the cost of their own lives.{{Fn|3c}}

==Aftermath==
{{main|Underground War}}
[[Image:BACArt.jpg|thumb|right|250px|The Word Bearers and Ultramarines battle in the Underground War beneath Calth's surface]]
More than one-half of the Ultramarines' muster (at least one hundred thousand Ultramarines, and thousands more soldiers) died during the Battle of Calth, and many more likely perished during the years of fighting in the Underground War that saw the scattered remains of the Word Bearers flee beneath the catacombs of Calth. However, the Word Bearers ultimately failed to eradicate the XIII Legion, and Guilliman lost little time in embarking his remaining forces to [[Terra]], to intervene in [[Horus]]'s planned overthrow of the Emperor. This was to prove decisive to the outcome of the [[Horus Heresy]]; although the Ultramarines were unable to reach Terra in time to take part in the [[Battle of Terra|battle]], word of their approach reached Horus, forcing him to gamble by allowing the Emperor to teleport aboard [[Vengeful Spirit|Horus's ship]].{{Fn|17}}

Erebus cared little for the military objectives of the Battle; he and his acolytes intended the destruction of Calth to be an offering to their [[Gods of Chaos|dark gods]], and, even as the Ultramarines regained the initiative, reckoned that the ritual had been a complete success. At the climax of the ritual, Erebus had summoned several of the most powerful [[daemon]]s which would take part in the [[Traitor Legion]]s' assault on the [[Imperial Palace]], and more importantly succeeded in summoning the massive [[Warp Storm]] known as the [[Ruinstorm]] that would effectively split the Imperium in two.{{Fn|2g}}{{Fn|2u}}{{Fn|2aa}} Meanwhile, Lorgar himself remarked that seeing the stoic Guilliman lose his temper made the whole undertaking worthwhile for him.{{Fn|2k}}

At some point during or after the Battle of Calth [[Argel Tal]] would remark to [[Khârn]] that there was another purpose to Calth. Lorgar and Argel Tal chose each Word Bearer that was sent to Calth based on their competence, specifically those who allowed their hatred of the Ultramarines to cloud their judgements and become unreliable. They were judged to be expendable to the Legion and sent on a mission it was expected very few, if any, would return from. Thus Calth served as a third purge for the ranks of the Word Bearers, removing those that were loyal but incompetent and ultimately a detriment to the Legion.{{Fn|7}} Millions of civilians died in the Word Bearers' cataclysmic attack, which permanently stripped away Calth's atmosphere and poisoned the Veridian sun's radiation, making it impossible for any life to survive aboveground without shelter. Thereafter, Calth's entire population occupied the vast cavern cities, with the exception of a few heavily shielded starports and surface cities.{{Fn|1}}{{Fn|6a}}

During the [[Great Scouring]], the Ultramarines, led by Captain Ventanus, presided over the destruction of the Word Bearers [[Adeptus Astartes Homeworld|homeworld]] of [[Colchis]].{{Fn|2ad}} In 999.M41, ten thousand years after the Heresy, Hol Beloth's [[Dark Apostle]], [[Maloq Kartho]], would revisit Ultramar as the [[Daemon Prince]] [[M'kar]], though he was unable to set foot on Calth, declaring the world "anathema" to him.{{Fn|6a}}{{Fn|6b}}

According to the official Ultramarines reckoning, the Battle of Calth has not yet ended; the [[Mark of Calth]] continued running after Guilliman's withdrawal from the system, and will continue to run as long as Lorgar or a single member of his Legion lives.{{Fn|2ad}}

==Forces==
===Imperium===
====[[Ultramarines]]====
*Majority of the [[Space Marine Legion|Legion]], totalling 185,923 [[Astartes]]:{{Fn|9a}}
**[[1st Chapter (Ultramarines Legion)|1st Chapter]]{{Fn|9a}}
***[[4th Company (Ultramarines Legion)|4th Company]]{{Fn|6c}}
***[[6th Company (Ultramarines Legion)|6th Company]]{{Fn|6c}}
**[[2nd Chapter (Ultramarines Legion)|2nd Chapter]]{{Fn|9a}}
**[[3rd Chapter (Ultramarines Legion)|3rd Chapter]]{{Fn|9a}}
**[[4th Chapter (Ultramarines Legion)|4th Chapter]]{{Fn|9a}}
**[[5th Chapter (Ultramarines Legion)|5th Chapter]]{{Fn|9a}}
**[[6th Chapter (Ultramarines Legion)|6th Chapter]]{{Fn|9a}}
**[[8th Chapter (Ultramarines Legion)|8th Chapter]]{{Fn|9a}}
**[[9th Chapter (Ultramarines Legion)|9th Chapter]]{{Fn|9a}}
**[[11th Chapter (Ultramarines Legion)|11th Chapter]]{{Fn|9a}}
***[[111th Company (Ultramarines Legion)|111th Company]]{{Fn|2c}}
***[[112th Company (Ultramarines Legion)|112th Company]]{{Fn|2c}}
**[[12th Chapter (Ultramarines Legion)|12th Chapter]]{{Fn|9a}}
**[[13th Chapter (Ultramarines Legion)|13th Chapter]]{{Fn|9a}}
**[[14th Chapter (Ultramarines Legion)|14th Chapter]]{{Fn|9a}}
**[[15th Chapter (Ultramarines Legion)|15th Chapter]]{{Fn|9a}}
**[[16th Chapter (Ultramarines Legion)|16th Chapter]]{{Fn|9a}}
**[[17th Chapter (Ultramarines Legion)|17th Chapter]]{{Fn|9a}}
**[[18th Chapter (Ultramarines Legion)|18th Chapter]]{{Fn|9a}}
**[[20th Chapter (Ultramarines Legion)|20th Chapter]]{{Fn|9a}}
**[[21st Chapter (Ultramarines Legion)|21st Chapter]]{{Fn|9a}}
**[[22nd Chapter (Ultramarines Legion)|22nd Chapter]]{{Fn|9a}}
**[[23rd Chapter (Ultramarines Legion)|23rd Chapter]]{{Fn|9a}}

====Loyalist [[Mechanicum]]====
*[[Legio Praesagius]] and elements of the [[Legio Oberon]] [[Titan Legion]]s, containing around 118 Titans{{Fn|9a}}
*[[House Vornherr]] - most of the [[Knight House]]{{Fn|9a}}
*[[House Tyrinth]]{{Fn|13}}
*35,000 Mechanicum personnel and [[Legio Cybernetica|Automata]]

====[[Imperial Army]]====
*1 Million [[Imperial Army]], [[Ultramar Auxilia]], [[Calth High Guard]], [[Calth Wardens]]{{fn|16}} and [[Calth Irregulars]] militia.
**[[Calth 5th Infantry Regiment]]{{Fn|15}}
**[[Numinus 61st]]{{Fn|2b}}

==='''Traitors'''===
====[[Word Bearers]]====
*Majority of [[Space Marine Legion|Legion]], totalling roughly 50,000 [[Astartes]]{{Fn|9a}}
**[[Asps of the Sacred Sands]]{{Fn|9a}}
**[[Black Comet]]{{Fn|9a}}
**[[Exalted Gate]]{{Fn|9a}}
**[[Flayed Hand]]{{Fn|9a}}
**[[Graven Star]]{{Fn|9a}}
**[[Inscribed]]{{Fn|9a}}
**[[Osseous Throne]]{{Fn|9a}}
**[[Third Hand]]{{Fn|9a}}
**[[Tri-fold Crown]]{{Fn|9a}}
**[[Twisting Rune]]{{Fn|9a}}
**[[Unspeaking]]{{Fn|9a}}
**[[Weeping Hand]]{{Fn|14}}

====[[Dark Mechanicum]]====
*[[Legio Suturvora]], with roughly 130 Titans{{Fn|9a}}

====[[Lost and the Damned]]====
**Roughly 500,000 [[Cultist]]s from the [[Recursive Kin and the Ring]], [[Brotherhood of the Knife]], and [[Mandari]]{{Fn|9a}}
**Traitor Imperial Army, including [[199th Thorion Yeomanry]]{{Fn|9d}}

==Trivia==
{{Trivia}}
===Conflicting sources===
*According to the novel ''[[Know No Fear (Novel)|Know No Fear]]'', the Battle of Calth concludes just short of twenty-four hours after Mark Zero, while the ground forces are evacuating the surface to escape the poisoned sun's radiation{{Fn|2aa}}; however, according to ''[[Garro: Oath of Moment (Audio Book)|Garro: Oath of Moment]]'', the 21st Company has been dug in "for days" following the initial Word Bearer Assault.{{Fn|5b}}

==See also==
*[[Known Battles of the Horus Heresy]]
*[[List of Battles of the Ultramarines]]
**[[Battle of the Macragge's Honour]]
*[[The Horus Heresy: Betrayal at Calth]]

==Sources==
*{{Endn|1}}: [[Codex: Space Marines (5th Edition)]], pgs. 13-14
*{{Endn|2}}: [[Know No Fear (Novel)]] - Dramatis Personae
**{{Endn|2a}}: ''Target//Acquisition'', Chapter 1, pg. 17-21
**{{Endn|2b}}: ''Target//Acquisition'', Chapter 2, pgs. 27-28
**{{Endn|2c}}: ''Target//Acquisition'', Chapter 3, pgs. 39-46
**{{Endn|2d}}: ''Target//Acquisition'', Chapter 6, pgs. 67-77
**{{Endn|2e}}: ''Target//Acquisition'', Chapter 7, pg. 92
**{{Endn|2f}}: ''Target//Acquisition'', Chapter 8, pgs. 98-106
**{{Endn|2g}}: ''Absolute//Overwhelm'', Chapter 1, pgs. 109-119, 120-121
**{{Endn|2h}}: ''Absolute//Overwhelm'', Chapter 2, pgs. 124-125, 136-141
**{{Endn|2i}}: ''Absolute//Overwhelm'', Chapter 3, pgs. 143-155, 160-163
**{{Endn|2j}}: ''System//Kill'', Chapter 2, pgs. 173-183
**{{Endn|2k}}: ''System//Kill'', Chapter 3, pgs. 185-201
**{{Endn|2l}}: ''Target//Engagement'', Chapter 2, pgs. 223-225
**{{Endn|2m}}: ''Target//Engagement'', Chapter 3, pgs. 227-238
**{{Endn|2n}}: ''Target//Engagement'', Chapter 4, pgs. 239-248, 251-256
**{{Endn|2o}}: ''Target//Engagement'', Chapter 5, pgs. 264-268
**{{Endn|2p}}: ''Target//Engagement'', Chapter 6, pgs. 273-278
**{{Endn|2q}}: ''Target//Engagement'', Chapter 7, pgs. 281-294
**{{Endn|2r}}: ''Target//Engagement'', Chapter 9, pgs. 295-302, 307-310
**{{Endn|2s}}: ''Target//Engagement'', Chapter 10, pgs. 312-319
**{{Endn|2t}}: ''Ushkul//Thu'', Chapter 1, pgs. 324-325, 329-335, 339-342, 342-344
**{{Endn|2u}}: ''Ushkul//Thu'', Chapter 2, pgs. 345-347
**{{Endn|2v}}: ''Ushkul//Thu'', Chapter 3, pgs. 353-360
**{{Endn|2w}}: ''Ruin//Storm'', Chapter 1, pgs. 365-370
**{{Endn|2x}}: ''Ruin//Storm'', Chapter 2, pgs. 376
**{{Endn|2y}}: ''Ruin//Storm'', Chapter 3, pgs. 377-385
**{{Endn|2z}}: ''Ruin//Storm'', Chapter 4, pgs. 387-399
**{{Endn|2aa}}: ''Ruin//Storm'', Chapter 5, pgs. 401-403
**{{Endn|2ab}}: ''Ruin//Storm'', Chapter 6, pgs. 405-408
**{{Endn|2ac}}: ''Un//Doing'', ''Frontispiece'', pg. 409
**{{Endn|2ad}}: ''Un//Doing'', Epilogue, pgs. 411-412
**{{Endn|2ae}}: ''Cover Art''
*3: [[Battle for the Abyss (Novel)]]:
**{{Endn|3a}}: Chapter One
**{{Endn|3b}}: Chapter Eighteen
**{{Endn|3c}}: Chapter Twenty
*4: [[The First Heretic (Novel)]]:
**{{Endn|4a}}: {{Cite Marker End}}, pgs. 39-62
**{{Endn|4x}}: {{Cite Marker End}}
*5: [[Garro: Oath of Moment (Audio Book)]]:
**{{Endn|5a}}: Track 1: Garro: Oath of Moment
**{{Endn|5b}}: Track 3: The Field of Battle
**{{Endn|5c}}: Track 11: Vantage Point
**{{Endn|5d}}: Track 12: Shoulder to Shoulder
**{{Endn|5e}}: Track 13: The Oath
*6: [[The Chapter's Due (Novel)]]:
**{{Endn|6a}}: Chapter Eight, pgs. 107-110
**{{Endn|6b}}: Chapter Twenty-Four, pgs. 313-314
*7: [[Betrayer (Novel)]]:
**{{Endn|7a}}: Chapter One
**{{Endn|7b}}: Chapter Eleven
*8: [[Honour to the Dead (Audio Drama)]]:
**{{Endn|8a}}: Track 1
**{{Endn|8b}}: Track 3
**{{Endn|8a}}: Track 13
*9: [[The Horus Heresy Book Five - Tempest]]:
**{{Endn|9a}}: pg. 39
**{{Endn|9b}}: pg. 51
**{{Endn|9c}}: pg. 133
**{{Endn|9d}}: pg. 251
**{{Endn|9c}}: pg. 93
**{{Endn|9f}}: pg. 32
*{{Endn|10}}: [[Aurelian (Novella)]]
*{{Endn|11}}: [https://battlebunnies.blogspot.com/search?updated-max=2016-02-08T12:55:00Z&max-results=10 Horus Heresy Weekender 2016 Presentation] ''(last accessed 6/27/2020)''
*{{Endn|12}}: [[Censure (Audio Drama)]]
*{{Endn|13}}: [[Warhammer: Visions 10]], pgs. 194-199 — ''Armies on Parade'' {{WHVisionsEnd}}
*{{Endn|14}}: [[Horus Heresy: Aeronautica Imperialis]], pg. 19
*{{Endn|15}}: [[Horus Heresy (game system)]],{{Cite Marker End}}
*{{Endn|16}}: [https://archive.ph/iem2w Warhammer Community: The Solar Auxilia Cohorts of the Horus Heresy Studio on Parade (posted 01/03/2024)] ''(saved archive page, dated 23 June 2024, last accessed 23 June 2024)''
*{{Endn|17}}: [[Echoes of Eternity (Novel)]], Epilogue

{{WHVisions}}

{{HHBattles}}

{{DEFAULTSORT:Calth, Battle of}}

[[Category:Battles (Adeptus Mechanicus)]]
[[Category:Battles (Collegia Titanica)]]
[[Category:Battles (Daemons)]]
[[Category:Battles (Dark Mechanicus)]]
[[Category:Battles (Horus Heresy)]]
[[Category:Battles (Knights)]]
[[Category:Battles (Ultramarines)]]
[[Category:Battles (Word Bearers)]]
[[Category:Ultramar]]

[[fr:Battle of Calth]]