Battle of Calth

The Battle of Calth, also referred to as the Calth Atrocity, was the name given by later Imperial scholars to the treacherous campaign conducted during the early stages of the Horus Heresy in the 31st Millennium by the traitorous XVII Space Marine Legion, the Word Bearers, on behalf of the Warmaster Horus against their hated rivals, the XIII Space Marine Legion, better known as the Ultramarines. The campaign was launched by the Word Bearers' Primarch Lorgar Aurelian with the goal of exterminating the XIII Space Marine Legion outright. The purpose of the Word Bearers' invasion of the Ultramarines' Realm of Ultramar in the Eastern Fringes of the galaxy was to tie down the XIII Legion and prevent them from reinforcing their fellow Loyalists as the Traitor Legions marched relentlessly on Terra itself. The crux of the campaign came on the Agri-world of Calth in the Ultramar Sector, where the Ultramarines successfully broke the Word Bearers' surprise assault after a viciously-fought siege action, though at the cost of terrible casualties and the complete destruction of Calth's atmosphere and once-verdant biosphere. As a result of the devastation wrought by the Word Bearers during the Calth Atrocity, future generations of Calth's people were required to live deep underground in massive subterranean hive cities to escape their world's radiation-scorched, airless surface. While both the Ultramarines and their Primarch Roboute Guilliman survived the Word Bearers' assault, the campaign successfully prevented the Ultramarines from participating in the Battle of Terra as Horus had planned.

Striking a Blow for Horus
During the opening days of the terrible conflict that would become known as the Horus Heresy in later years, the Warmaster Horus launched his treacherous attack on the world of Istvaan III against the Loyalist elements within four of the Space Marine Legions that turned to Chaos, including the Sons of Horus, the Emperor's Children, the World Eaters and the Death Guard. It is believed by Imperial savants that the Loyalists made up approximately one-third of the combatants at Istvaan III. Having successfully purged the remaining Loyalists from the Traitor Legions, Horus plotted the next stages of his insurrection against his once-beloved father, the Emperor of Mankind. When word reached Terra of the Warmaster’s treachery at Istvaan III - a result of the flight of the frigate Eisenstein under the command of Captain Nathaniel Garro and a small group of Loyalist Space Marines from the Death Guard Legion - the Emperor ordered the deployment of seven Loyalist Astartes Legions to the Istvaan System to bring Horus to account for his actions.

While Horus made his plans for what would become known as the infamous Drop Site Massacre on Istvaan V, the Warmaster sent word to the XVII Legion’s Primarch Lorgar that the time had come for his Astartes, the Word Bearers, to strike against the Imperium. The Warmaster was keenly aware of the bitter hatred that Lorgar had for his Primarch brother Roboute Guilliman and his XIII Legion, the Ultramarines, who had once humiliated the Word Bearers by destroying their city of Monarchia on the world of Khur at the Emperor's orders during the Great Crusade. The Ultramarines had taken no pleasure in this act, which was intended to teach Lorgar and his Astartes to adhere to the atheistic doctrines of the Imperial Truth rather than spread the false belief that the Emperor was divine to all the worlds that they conquered. Yet Lorgar and the Word Bearers had never forgiven the Ultramarines for this action and they longed for vengeance against the XIII Legion.

Horus told Lorgar that he had fed Guilliman false intelligence in regard to a possible threat within the Veridian System in the Segmentum Tempestus, far to the galactic south of Terra. This supposed threat stemmed from the Orks of the Ghaslakh Empire. Horus had ordered the XIII and the XVII Legions to muster and meet at the world of Calth in the Ultramarines' Realm of Ultramar, in order to conduct a massive joint campaign of extermination against the Ghaslakh xenohold, a common mission for the Astartes during the final days of the Great Crusade.

It would be at Calth that Lorgar would launch a surprise attack on the Ultramarines whilst they were gathered for the campaign against the Orks of Ghaslakh. The XIII Legion would be caught completely unaware while the Word Bearers would use the advantage of surprise to completely annihilate their hated rivals. The assault at Calth would also allow the Word Bearers to reveal that they, too, now served Ruinous Powers. Calth was not chosen as the site of the confrontation between the Word Bearers and the Ultramarines by chance, for the Word Bearers intended to destroy one of the jewels in the Ultramarines' realm of Ultramar (then known as the Ultramar Coalition), just as the XIII Legion had destroyed one of the Word Bearers' greatest achievements, the sacred city of Monarchia, four decades earlier.

Horus ordered the majority of the XVII Legion to Ultramar, and the dark powers of the Warp gave them sure and swift passage across the increasingly restless Immaterium.

As the Word Bearers entered Ultramar space, Lorgar prepared his Legion for the inevitable slaughter that would follow. Command of the main assault force was given to Kor Phaeron, the First Captain of the XVII Legion and one of Lorgar’s most favoured champions. Calth was to be Kor Phaeron’s operation to execute, far more than it was Lorgar’s. Kor Phaeron had planned the assault of Calth for his Primarch meticulously, and executed it with the aid of the Dark Apostle Erebus. The punishment and annihilation of the XIII Legion was the campaign's principal aim; the humiliation and execution of Lorgar's hated rival Roboute Guilliman was a secondary objective. But for Lorgar, the assault would also mark his first opportunity to gain true favour in the eyes of the Dark Gods he now served; to prove to them that he had earned his place as their Chosen One.

The first phase of the Word Bearers' attack plan involved the capture of a vessel of the Ultramarines' fleet. The Word Bearers waylaid the fleet tender Campanilewhich was at the outer rim of the Calth System, boarded it and seized control. The Traitors intended to use the captured vessel to strike a crippling surprise blow against the Ultramarines' massed fleet that would mark the start of their vicious assault.

The Calth Conjunction
Calth was a verdant Agri-World in the Veridian System of Ultramar whose manufacturing output at the time of the Horus Heresy would have rivalled that of Macragge in only two to three more decades. There was even talk of constructing a superorbital ring like that which encircled Terra and was intended to get cargo to and from orbit as quickly and inexpensively as possible. The most important and heavily populated worlds of the Imperium had all constructed similar rings, which were marvels of human technology and engineering and symbols of the new Golden Age that the Emperor intended to create for Mankind. Calth intended to join Macragge, Saramanth, Konor, Occluda and Iax as the most important worlds of the Ultramar Sector, whose influence already extended across a vast swathe of the Imperium's Segmentum Ultima. Calth's people hoped that their world would become one of the anchor points of the glorious new Imperial civilisation that would come to full fruition after the end of the Great Crusade.

Roboute Guilliman and a sizeable portion of the XIII Legion had been stationed near the moons of Saturn in the Sol System when they received their orders to mass at Calth from Horus. Some within the Primarch’s command questioned the need for the deployment of so large an Astartes force; the size of the combined Ultramarine / Word Bearer force would have been the largest since the Ullanor Crusade. The Ultramarines' officers had seen the tactical audits and knew that the Orks of Ghalaskh presented little real threat to the Imperium. But the wiser, more experienced Chapter Masters realised that the Ultramarines were to lend a little shine to the ungainly reputation of the Word Bearers by operating in concert with them, and the assault on Ghalaskh was intended to demonstrate the authority of the Warmaster over all of the Astartes Legions. Regardless of the Ultramarines' trepidations or personal feelings, Roboute Guilliman acquiesced to Horus’ rightful authority and moved his Legion to the world of Calth in order to rendezvous with other elements of the Ultramarines and take on supplies for the coming campaign. The Ultramarines would then join with the Word Bearers and combine their efforts to crush the Greenskins. In truth, Guilliman still had misgivings about the actions that the Ultramarines had been ordered to take by the Emperor against the XVII Legion during their chastisement at Khur. Guilliman felt that the Emperor had been wrong to use the Ultramarines as an instrument of humiliation against a brother Legion and as an example of Astartes perfection. Guilliman was correct in believing that relations between the Ultramarines and the Word Bearers had been poisoned by what had happened on Khur, but wrongly thought that Horus had ordered the two Legions to work jointly in the pacification of Ghalaskh in an attempt to close the rift that had opened between them.

Signs and Portents
There were many portents of the tragedy soon to unfold upon Calth. Given the extraordinary thoroughness with which the XIII Legion maintained its readiness, it might have been considered tragic, or incompetent, that so few were heeded. The first signs of the Word Bearers betrayal was the minor interruption in Vox traffic around Calth that was attributed to solar distortion – the void was known to forever creak and whisper around the audible and electromagnetic ranges. Soon there were various reports of a voice chanting over Calthian Vox-links in Calth space. This eerie chanting soon interfered with the main orbital data-feed between the orbital installations and the surface of Calth for several seconds. An hour later, there were two more bursts of such interference, the source of which remained undetermined. Calth Communications Control reported an hour later that there were "a series of malfunction events" and warned that further communication disruptions might be expected during the day until the problem was identified. An hour after that, on the night side of Calth, the first of the bad dreams began.

There were psychic portents as well. Hundreds of Ultramarines Battle-Brothers had been Librarians before the Emperor's decree at the Council of Nikaea banned the Space Marine Legions' use of psykers. Many Astartes Librarians of the Legions resented the Decrees of Nikaea, but honoured their oath to the Emperor and surrendered their Librarian panoply and wargear, returning to the lines as ordinary Battle-Brothers. Before the attack on Calth, those gifted with psychic abilities suffered severe headaches and pain behind the eyes. They ignored the pain, chalking it up to nothing more than fatigue after having gone without rest for several days during the preparation phase for the coming campaign against the Orks. It had not been possible to shut down higher mental functions and sleep, or at least remedially meditate. This mysterious pain was actually a warning sign broadcast through the Warp of the impending approach of the Forces of Chaos. The former Librarians all ignored the headaches. Few would survive the coming assault long enough to regret it.

For days the Word Bearers' Chaos Cult echelons chanted monotonously into the ether as they conducted sacrificial rituals intended to bring forth the entities of the Warp to supplement the forces of the XVII Legion in the battle to come. Eight names were now repeatedly and constantly broadcast into the data flows of Calth's global Cogitator network. No data filter or noospheric barrier would block the repeating code or erase it, because it was only composed of regular characters. They were neither toxic code nor viral data, but once they were inside the system, and especially once they had been read and absorbed by the Mechanicus’ noosphere, they began to grow in size. Soon these names became combinations of letters and then longer, deeper meanings essentially a form of runic Chaotic sorcery. These implanted meanings were caustic, infectious and indelible. There were eight of them, the sacred number of Chaos – the Octed.

At the orbital watchtower high above Calth, located at the Kalkas Fortalice, Uhl Kehal Hesst, a Mechanicus Server of Instrumentation, detected scrapcode within the global Cogitator system – the dull amber threads of diseased information buried in the mass of healthy data. There was 2% more of it than any Analyticae projection had calculated for the Calth noosphere, the entirety of Cogitator data being used across Calth's global system, even under the irregular circumstances wrought by the conjunction of Imperial forces on the planet below. This was an unacceptable margin of error for the system, and the Mechanicus' senior Tech-priests reported the problem to Guilliman. They informed the Primarch that the scrapcode problem had been identified as a hindrance and that, though regrettable, these things did often happen within complex Cogitator systems and should not hamper his Legion's preparations for the coming campaign.

The Offering
Multiple Word Bearer vessels had already arrived in the Calth System, which included at least a half dozen or so crimson-coloured vessels, unusual for the Word Bearers, since the XVII Legion's colours were a flat steel grey. Counted amongst their number was the massive Infidus Imperator – a Grand Cruiser and the flagship of the Word Bearers' First Captain Kor Phaeron, as well as Destiny’s Hand, the Battle-Barge of the Dark Apostle Erebus. The Dark Apostle was tasked with a vital mission on behalf of the Warmaster. While the rest of the XVII Legion’s vessels stationed themselves to the rear of the massive orbital flotilla gathering above the world, Erebus teleported down to Calth’s surface, to the Satric Plateau, located 2,000 kilometres north of Numinous City. Covered in frost from the hard winter, this particular area was selected as the best potential site to conduct the blasphemous Chaotic ritual Erebus had been tasked to perform, for the Immaterium vectors aligned perfectly and the barrier between realspace and the Empyrean was particularly thin in that part of Calth.

A Word Bearers strike team already on Calth awaited the Dark Apostle. Its leader was Essember Zote, one of the dreaded daemon-possessed Word Bearers known as the Gal Vorbak. He had with him a work party of the Tzenvar Kaul, “The Recursive Family”, one of the many Chaos Cults serving the XVII Legion. This was to be the point of the true conjunction on Calth. The men of the Kaul laid out a circle of polished black rocks upon the ground, each taken from the volcanic slopes of Istvaan V and marked with a blasphemous sigil. These were arranged in a perfect circle a kilometre in diameter. The rocks were summoning stones, whose latent Chaotic power could make one sick just by holding them.

The men of the Tzenvar Kaul approached the summoning circle, each carrying other offerings from the Istvaan System. Marching in procession, they bore along portable stasis flasks, the fluid within murky with blood, as they contained Progenoid Glands and gene-seed harvested from all the Legions that had fought each other on Istvaan III and Istvaan V, Traitor and Loyalists alike. The Kaul carried the flasks into the circle and the moment that they crossed the mystical border created by the summoning stones, the bearers started to whimper and retch. Several passed out, or suffered strokes, and fell, smashing the flasks. As the moon rose into perfect alignment, Erebus took his Vox-link and sent a message to the Traitors in orbit. The time had come for the Word Bearers to have their revenge against the Ultramarines and their insufferable Primarch.

Betrayal
"My brother, hear me. Warriors of the XVII 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."

- Vox transmission, Primarch Roboute Guilliman - Battle of Calth, Mark 0.00.01

Aboard the docked Ultramarines vessel Samothrace in high orbit of Calth, Captain Sorot Tchure of the Word Bearers was enjoying a formal dinner arranged by his Ultramarines counterpart and friend, Honorius Luciel, the Captain of the 209th Company. Receiving his order to carry out his part of the plan, Tchure returned back to the dining table with his fellow Word Bearers. As the two Astartes officers struck up a conversation, the Ultramarines captain was aware that his counterpart seemed agitated.

Tchure shared with Luciel that he had recently learned a bit of useful warcraft whilst fighting in the Istvaan System that he thought his Ultramarines counterpart would appreciate. Unaware of any recent Imperial campaigns within the Istvaan System, Luciel's interest was piqued. The Word Bearers officer spoke of the art of betrayal and its inherent power because the treachery ran so deep, especially if it was committed against a trusted ally or close friend. Asked to join the advance into the Ultramar System, he told Luciel that he had to prove his commitment. The Word Bearer subtly tried to warn Luciel that in order to show that he was loyal to the XVII Legion’s new cause he must first betray his friend. Realising, too late, the meaning of Tchure’s obscure warning, Luciel rose to his feet to defend himself, but was cut down with a well-placed plasma shot through his chest. Tchure and the rest of his Word Bearers then opened fire upon the other startled Ultramarines at the dinner, killing Luciel's entire entourage, the heart of the leadership of the 209th Company. They were than offered as a sacrifice to the Ruinous Powers -- the first casualties of the Battle of Calth.

Aboard the bridge of the Samothrace, an alarm alerted the Officer of the Watch of a malfunction. Accessing his Cogitator for clarification, he was shocked when he saw the phrase: weapons discharge, company deck. Activating the vessel's internal Vox system and rousing deck protection, the Officer of the Watch began to cordon and secure the entire deck used to house the 209th Company. Suddenly, general quarters was sounded, as the bridge came alive with a cacophony of various alarms, klaxons and bells all screeching and booming; the proximity alarm, the collision warning alert, the course defect advisor as well as many others. Looking at the main screen for visual verification, the Officer of the Watch saw a starship heading at full collision speed for the heart of the Ultramarines fleet gathered above Calth.

Opening Gambit
The captured tender Campanile had crossed the inner defence ring of Calth, its codes accepted as authentic by the orbital defence grid. The mass of the Ultramarines' fleet lay in its path as well as Calth's massive orbital dock yard complex. As it passed within the orbit of Calth’s moon, it began an abrupt acceleration. The captured vessel screamed into Calth’s orbit like a streaking projectile fired from a bolter, heading directly for the Calth Fleet Yards and the fledgling infrastructure of the planet’s first proper superorbital ring. It somehow managed to miss some of the larger vessels, grazing them with its Void Shields and skimming the surface of others. Smaller craft that lay in the Campanile's path were utterly annihilated; cargo boats, lighters, ferries, maintenance riggers. The Ultramar Azimuth Graving Dock was vaporised by the runaway vessel. Several Ultramarines vessels suffered catastrophic hull damage and multiple orbital manufactory modules were destroyed, instantly killing thousands of Imperial Artificers and engineers. The Campanile lost none of its momentum and continued with its suicidal orbital plunge.

Still moving, the captured vessel punched through a hollow construction spheroid that housed three separate vessels, obliterating them all. The assembly ruptured and the debris was propelled into the attached habitat modules, voiding them into space. The devastating trajectory of the Campanile caused the Calth Veridian Anchor to shudder under the onslaught. Internal explosions occurred throughout the structure as even more starships in various states of disrepair were destroyed or catastrophically damaged. Larger Battleships exploded as their power plants and vast ammunition stockpiles were critically compromised. Huge, burning sections of the orbital yards' superstructure as well as the debris of destroyed vessels rained down upon the surface of Calth. The Ultramar Zenith Graving Dock suffered integral gravimetric failure and dropping out of orbit, twisting and breaking as it plummeted to the planet below. The massive Grand Cruiser Antrodamicus, supported by the dock, ripped free of its moorings and slid backwards, caught in the gravity of Calth. With its drives off-line it had no power to stabilise itself or prevent its inevitable doom.

The Campanile continued to plow through the orbital installations surrounding Calth, shooting forward like a solid projectile, a gargantuan mass of streaking death. It annihilated a pair of slipways and the ships within them, ramming through the vital data-engine hub and destroying multiple data-engines as well. As the automatics failed and the noosphere experienced a critical and fatal interrupt, the orbital fleet yard’s power core was obliterated, killing over 35,000 men and women. The Campanile finally broke up, still travelling at immensely high realspace velocities, and a large chunk of the disintegrating vessel spun out of control and destroyed another Ultramarines Battleship. The remaining pieces of the Campanile cleared the far side of the Calth Veridian Anchor and rained down, burning like meteorites, upon the surface of the planet.

Aboard the Samothrace, Sorot Tchure and his Astartes killed most of the ship’s crew. Advancing to the main bridge, burning through blast hatches that had been closed in desperation, Tchure confronted the ship’s captain, who solemnly announced his disinclination to assist the traitorous Word Bearer. Ignoring the officer, Tchure brutally killed him, letting the body drop to the deck. The Word Bearers then proceeded to coldly murder the entire bridge crew. The Battle of Calth had begun in earnest, as the Word Bearers and their Chaos Cultists began forming up in their thousands for the brutal assault upon the surface.

Assault at High Anchor
The vast orbital shipyard that was the Calth Veridian Anchor was damaged beyond the possibility of salvation or stabilisation by the ruinous trajectory of the Campanile. With the destruction of Calth's main data-engine hub the Mechanicus' ability to communicate with all of its various installations across the world' orbital plane had been torn out, their systems compromised beyond repair. All the various orbital stations that rode at Calth's high anchor had been shattered beyond repair, consumed by firestorms from within when enough atmosphere remained to fuel the blazes. The Ultramarines fleet and Calth's orbital fleet yard infrastructure had suffered a grievous injury while the death toll amongst Imperial personnel and Calthian civilians in the orbital habitats was catastrophic. In the first few seconds after the initial impact of the Campanile, starships across the high anchorage attempted to desperately power up their drives and bring their weapon systems back online. Some attempted to generate enough power in the vain hope of raising their Void Shields or to slip from their moorings in order to reposition in case of an attack.

Then the massive Grand Cruiser of the Word Bearers Legion known to the Ultramarines as the Raptorous Rex, but which the Word Bearers had renamed the Infidus Imperator (“False Emperor”) to display their new allegiance opened fire. Kor Phaeron ordered the massive warship that was his flagship to discharge all of its primary Lance weapons at the Battle Barge Sons of Ultramar, obliterating it in one brutal salvo. In formation behind the massive Word Bearers flagship, other warships of the XVII Legion's fleet, including the Crown of Colchis, the Battleship Kamiel, the Flame of Purity, the Spear of Sedros and the Battle Barge Destiny’s Hand, all opened fire upon the remains of the Ultramarines fleet riding at high anchor.

Shipmaster Ouon Hommed, the captain of the Ultramarines' heavy destroyer Sanctity of Saramanth, saw the Infidus begin its merciless prowl along the anchorage's line of docked Loyalist vessels. He understood that the vessel was executing starships the way a man might execute a row of helpless prisoners. The Sanctity was sitting at anchor with its drives cold and it would take at least 50 minutes to rouse the ship to operational readiness, a reality for every starship in the fleet of the XIII Legion which had been sitting cold at high anchor for the conjunction with the incoming Word Bearers. All of their power plants were at lowest yield for the purposes of maintenance, loading and embarkation checks with their drives, weapons and shields off-line, leaving them all under the protective aegis of Calth's planetary defence grid. The Word Bearers continued their onslaught of the Loyalist fleet unabated, leaving Roboute Guilliman to witness the perfidy of his brother Lorgar first-hand.

From Bad to Worse
Shortly after the Campanile exploded in orbit, the datashock from the resulting scrapcode onslaught killed Mechanicus Server Uhl Kehal Hesst through a vicious neural biofeedback mechanism commonly known as datashock amongst Tech-adepts (more officially termed hypertraumatic inload syndrome). Weapons fire hit the orbital watchtower at Kalkas Fortalice a nanosecond after the shock wave of data was unleashed by the Word Bearers' coordinated assault. The data noosphere of Calth immediately collapsed. The tower’s manifold field stuttered out. Hesst felt and absorbed the shared neural agony of several thousand deaths: his Mechanicus brethren aboard the primary shipyard, aboard the docked vessels, and in the tower around him as they died and their agony was transmitted from their dying neural cortexes through their augmetic systems into the noosphere they had been connected to at the time of their deaths. With the death of Hesst and his fellow Mechanicus personnel, all of the networked Cogitator systems of Calth went off-line. Calth’s planetary defence grid ceased to function, leaving the entire Ultramarines fleet completely unable to mount any defence against the Traitor warships' attack.

Fortunately Hesst’s second-in-command, Meer Edv Tawren, a Magos of Analyticae, was not killed by the infected scrapcode. It was basic Mechanicus data server protocol that saved her life, for it required a server's second-in-command to unplug his or her neural interface from the data noosphere when a significant scrapcode assault was under way so that there was no danger of the deputy also being compromised by the infected data. This operational safety measure saved Tawren from far more than just a scrapcode infection. When Hesst died in her arms, he charged his deputy to connect herself to a working server to attempt the reconstruction of the noosphere and bring the orbital defences back online. Magos Tawren would play a pivotal role in the coming conflict.

Debris began to fall from the clouds, trailing fire like meteorites, raining across the fertile river valley of Kalkas Fortalice. Some of the heavier pieces of falling debris struck buildings, exploding them like artillery strikes. The hail of debris had only begun as larger objects begin to fall – parts of ships, orbitals and the docking yards. Magos Tawren saw the unfolding disaster before her sensors did. She saw the Grand Cruiser Antrodamicus falling backwards into the atmosphere, stern first, towards Kalkas Fortalice.

Meanwhile, unbeknownst to the Ultramarines, Kor Phaeron materialised aboard the Samothrace through arcane means. The Word Bearers used the Samothrace to slip past the Ultramarines into the gates of the Zetsun Verid Fleet Yard. Behind it, Calth’s main orbital shipyard continued to burn. No one challenged the Samothrace, since it was an Ultramarines vessel running for cover and Vox and noosphere communications amongst all of the orbital installations was completely dead. No one aboard the orbital habitats within the Zetsun Verid Yard questioned the fact that their yard's infrastructure had not been targetted by the Traitors even though all the fleet yards around it had been obliterated. The Samothrace pulled into the ship docks between two Escort vessels sheltered in the yard space. The Ultramarines put up intense resistance, but eventually the Zetsun Verid Yard also fell to the Word Bearers. The Traitors' only setback was that the destruction of Calth's planetary data noosphere was so complete, it took the Word Bearers' Dark Mechanicus allies longer than expected to reconstruct a workable planetary data network. The reason the Traitors had spared the Zetsun Verid Fleet Yard was that it contained an advanced data-engine hub capable of substituting itself, in the event of an emergency, for the primary data hub of the Calthian noosphere located at the Calth Veridian Anchor.

The Dark Mechanicus Tech-adepts brought the Zetsun Verid Yard systems online and its data-engine resumed normal operations. Sensing that the planetary weapons grid was inactive, and that the inactivity had been caused by the inexplicable loss of the data-engine hub located aboard the Calth Veridian Anchor, the Zetsun Verid data-engine automatically obeyed its standard protocols and assumed control over the planetary data network and the planetary defence system, bringing the Calthian weapons grid back online. The Word Bearers were now in control of the effective firepower of a major Imperial battlefleet. The firepower of this defence system was distributed across the surface of Calth and its orbit. Kor Phaeron had his Dark Mechanicus Magi bring online the 962 orbital defence platforms as well as several ground-based stations, including the weapon pits located at Calth's poles. The Calthian planetary weapons grid was capable of keeping at bay an entire expeditionary fleet or primary battlegroup and it soon began to fire on the orders of the Word Bearers, assaulting all of the neighbouring planets and spaceborne Imperial facilities in the Veridian System.

The Ultramarines fleet had scattered, having been reduced to about a fifth of its original strength by the Word Bearers' surprise attack. Those vessels that remained either fled to the far side of the Veridian star to avoid an attack by the Word Bearers' fleet and the inexorable fire of Calth's defence grid or, like Primarch Guilliman’s flagship, the Macragge’s Honour, lay helpless and drifting in Calth's high orbital anchor zone. There were virtually no warships left for the Ultramarines to fight the Traitors in the void. It was simply a matter of time before the Word Bearers picked off the remaining warships of the XIII Legion's fleet. Next, the Calthian defence grid destroyed the local Forge World of Verida Forge, a small moon with offensive capabilities. It then assaulted a Star Fort near the Veridian System’s Mandeville Point and several more Loyalist capital ships, vaporising them all.

The Ultramarines' 1st Chapter Commander, Marius Gage, was the first amongst the Loyalists to recognise the attack pattern of the Word Bearers’ fleet. Many of the surviving Ultramarines starships were the largest and most powerful capital ships in the XIII Legion's fleet. With the Traitors firmly in control of Calth's planetary weapons grid the Word Bearers could easily pick off the most serious threats at their leisure. Those ships that had been spared were all helpless and drifting, like the Macragge’s Honour. The moment they shook off the effects of the scrapcode assault, began to start up their drives, or raised their Void Shields, Calth's weapons grid would target and destroy them. The Word Bearers intended to take as many of the XIII Legion’s capital ships intact as they could to bolster their own fleet and enhance their overall striking power in the longer conflict to come against the Loyalists.

Calth Besieged
"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."

- Vox transmission, Primarch Roboute Guilliman - Battle of Calth, Mark 1.57.42

The savagery and perfidy of the Word Bearers’ initial orbital assault left the Ultramarines and their allies reeling in shock. Their fleet had been decimated and was scattered across the Veridian System. Entire worlds within the system had been destroyed. Hundreds of thousand Chaos Cultists infiltrated onto Calth by the Word Bearers in the months before the battle began as part of the build-up for the assault on the Orks of Ghalaskh attacked their former Loyalist allies in the Imperial Army, attempting to destroy them in the name of the Chaos Gods. The forces of the Word Bearers involved in the invasion of Calth numbered over 100,000 Astartes and included the traitorous elements of the Dark Mechanicus and fearsome Traitor Titans. Across the planet a brutal ground war soon erupted. The savagery of the Word Bearers’ assault threatened to overwhelm the beleaguered defenders within only a matter of hours. However, Kor Phaeron had underestimated the tenacity and resolve of the Ultramarines.

At Barrtor, two Ultramarines captains of the 111th and 112th Companies decided to consolidate their commands with the forces of the Imperial Army and the Mechanicus in their region of the planet and pull back eastwards into the Sharud Province, since if they did nothing, they would be killed by the heavy debris falling from orbit following the Word Bearers' destruction of the orbital installations. Captain Phrastorex of the 112th decided that he would meet the Word Bearers' vanguard to determine what terrible accident had occurred in orbit while Captain Ekritus of the 111th would lead both Ultramarines companies into Sharud. Taking only a detachment of his Honour Guard with him, Phastorex walked down the slope to the flood plain where the local forces of the XVII Legion had finally arrived on-planet. Phastorex observed to his surprise that the Traitors already had their tanks and their Astartes formed up as if to begin an assault. As Phrastorex raised his hand in greeting to his fellow Astartes, the Word Bearers opened fire. Phastorex and his men were massacred by the Word Bearers before they could even comprehend such an act of base treachery. Captain Ekritus witnessed the massacre, and was eager to visit his vengeance upon the Traitors, but was restrained from rushing to his doom by his Battle-Brothers. Then, again without warning, the Word Bearers' Traitor Titans began to open fire on their position.

The shattered remnants of the 111th and 112th Companies retreated into the burning woods of the forest located east of the Boros to escape the Traitors' assault. The Word Bearers had coloured their Power Armour and their vehicles in the new crimson Legion colour scheme chosen in place of the XVII Legion's old steel-grey colours to herald their change of allegiance. It would not be long before the Ultramarines took to calling this new colour scheme "Traitor's Red," a moniker the Word Bearers relished. They had also adorned themselves with the abominable iconography of their new faith in Chaos, which their Loyalist counterparts saw as only further evidence of their corruption. The Word Bearers' Achilles and Proteus Pattern Land Raiders led their attack upon the Loyalist forces retreating before them. Captain Ekritus managed to find a position above the treeline from where he could see the vast hosts of the Word Bearers and their auxiliaries assaulting the Calthian towns along the river and the port of Boros. There were tens of thousands of Chaos Cultist soldiers, Battle Groups of Chaos Titans and endless phalanxes of crimson-armoured Chaos Space Marines. Ekritus watched in horror as the treacherous XVII Legion’s assault swept aside everything in its path.

There were bodies everywhere as the Word Bearers advanced without remorse against their unsuspecting victims; civilians, troops of the Imperial Army, and far too many Ultramarines. Men lay dead with their weapons still sheathed or covered, cut down without the opportunity to face their deaths. Heaps of cobalt-blue Power Armour that contained limp Astartes corpses lined the roadways and arterials. Some had been stacked against fences and walls like firewood whilst others had been cut open and emptied. A few had been nailed to posts, or against the sides of buildings. Some appeared to have been butchered or even…eaten. This was not just war, but desecration. It defied and shamed the codes and precepts of all the Astartes Legions, traditions that had been set down by the Emperor himself. The Word Bearers had perverted not only the laws and traditions of the Imperium, but the basic moral code of Mankind.

Desperate Hope
The Grand Cruiser Antrodamicus survived its unpowered fall through Calth's atmosphere from orbit largely intact, but its blazing bulk demolished a large swathe of the once teeming city of Kalkas Fortalice, clearing a patch two and a half kilometres wide. Meteoric debris continued to rain down from the sky all around it, bombarding the city and the surrounding landscape. Fortunately, Magos Tawren managed to flee the destruction of the Watchtower before the large vessel struck the planet’s surface. Little information about what had occurred in orbit was available to the Loyalist survivors on the planet’s surface. They knew that there had been some major incident in orbit that was raining destruction down on Calth, but whether it was an accident or an attack remained unclear. But with the orbital fleet yards gone and all Vox traffic scrambled, there was no way for the remaining Loyalist units on Calth's surface to communicate with one another or with Ultramarines command.

Eventually, Magos Tawren and her Loyalist Skitarii forces managed to consolidate with the surviving Ultramarines of the 4th Company, including their commander, Captain Remus Ventanus. Assessing their desperate situation, Ventanus realised he had a vast horde of Chaos Cultists arrayed against his forces as well as hundreds of thousands of Traitor Astartes. The Traitors also possessed multiple Battle Groups of massive Traitor Titan war engines within their ranks. Magos Tawren suggested seeking refuge at Leptius Numinus, the old Imperial gubernatorial palace on the plains of Calth. Leptius Numinus possessed a non-active but functional data-engine as well as a high-cast Vox transceiver array. Though not currently operational, this equipment had been scrupulously maintained, and because the systems at the palace had been off-line when the Word Bearers began their assault, they might have been spared the scrapcode infection and electromagnetic pulse damage that had brought down networked communications all over the world. If the 4th Company and its allies could reactivate the data-engine at Leptius Numinus, they might be able to contact what remained of the XIII Legion's fleet in the Veridian System.

This plan proved successful and the Loyalists reached the old palace. The commander of Tawren’s Skitarii suggested that they utilise their dedicated emergency manifold to power up the palace's data-engine and Vox assembly and contact other possible surviving Ultramarines, Imperial Army or Loyalist Mechanicus forces. Ventanus eventually established short-range communications with other besieged Ultramarines units. His situation was not unique. All of the Ultramarines on Calth's surface found themselves mired in the same predicament. None knew what had befallen their Primarch, Roboute Guilliman, or whether he was still alive.

Even as the 4th Company converged on Leptius Numinus, a large horde of Traitor forces commanded by the XVII Legion's Commander Morpal Cxir had already encircled the palace and launched an attack against its small group of valiant defenders. The Word Bearers used their vast hordes of Chaos Cultists as human fodder, driving them forward as they protected themselves from any significant counterattack. Terrible carnage was inflicted upon the Loyalist defenders by packs of horrific daemons summoned from the Empyrean as they punched through any breaks within the defensive line. As the assault grew more fierce, the defenders of Leptius Numinus prepared to sell their lives dearly, but they were spared from that dark fate by the timely arrival of the 4th Company and its heavy armour support. The reinforcements bolstered the defenders' resolve as they tore into the ranks of the Traitors and decimated their ranks. In the meantime, Tawren attempted to reactivate the data-engine and establish full communications with the remains of the Loyalists' fleet. Yet, Vetanus knew that although they were strong in spirit and well-armed, their forces could not hope to repel the Traitors' attacks indefinitely. It was only a matter of hours before the defenders' position would be overrun and everyone was slaughtered.

Despite the brutality of the Word Bearers' assault, Tawren focused on her duties and managed to reactivate the dormant data-engine. With it, she was able to gather a clearer picture of the situation, especially the truly catastrophic scale of the losses: the enormous death toll, the systematic annihilation of the Ultramarines’ fleet, millions slain and entire cities aflame as burning debris plowed into the surface of Calth from orbit. Ventanus finally managed to contact Roboute Guilliman and provide him with a picture of what remained of the Loyalist ground forces on Calth. As many as 30,000 Ultramarines Battle-Brothers and 200,000 Imperial Army and Mechanicus warriors were still active in hundreds of scattered pockets of fierce resistance. Coordinated, they could achieve more than if they continued to fight alone and unsupported. In any other circumstances, Calth would have been declared lost to the Traitors, but the XIII Legion's fierce resolve prevented them from giving into despair. Roboute Guilliman believed that the situation could be salvaged and the Word Bearers made to pay a price that they had not expected.

Desperate Gambit


Fortunately, amidst all this negative information, somehow Magos Tawren managed to discover a killcode that her former mentor Hesst had hidden within a secure data-engine at the Zetsun Verid Fleet Yard that had then been closed off from the rest of Calth's planetary network. This data-engine was the manifest Cogitator of the Cargo Handling Guild, which was located in a secured bunker in the industrial zone between Numinus Starport and the Lanshear landing grounds. It was responsible for running cargo operations for both Numinus and Lanshear, and thus was more than powerful enough to manage the data load of the planetary weapons grid. As a civilian Cogitator, it was not a primary military target during the Word Bearers' initial assault and Hesst had been able to take advantage of this before his death. Though he had suffered from the painful overload of the data-shock that was killing him, Hesst had managed to clean this data-engine with his killcode and then keep it severed from the broader planetary network. Contacting Guilliman aboard his flagship, the Macragge’s Honour, Ventanus apprised his Primarch of the new developments. He informed him that the enemy was controlling Calth's planetary defence grid using a captured data-engine on one of the surviving orbital platforms at the Zetsun Verid Yard. Magos Tawren would be able to purge the system of the Word Bearers scrapcode using this Cogitator, but could not access the Cargo Handling Guild's data-engine while the Word Bearers remained in control of the platform. They would need assistance from the Ultramarines in orbit in order to gain control of the platform and perform the purge of the planetary weapons grid.

Several plans were debated aboard the Macragge's Honour by the Ultramarines' commanders. Shipmaster Hommed recommended a ranged bombardment of the platform using the flagship's main weaponry. The Macragge’s Honourcertainly possessed the needed firepower. Chapter Master Marius Gage seconded the suggestion. But if they did not make a direct kill of the Word Bearers holding the platform with the first salvo, there was a real danger that the enemy would retaliate using the planetary defence grid and finish off the Ultramarines' flagship. Chapter Master Klord Empion of the 9th Chapter opted instead for a close attack plan: bring the flagship back to full power, throw off the enemy Cruisers surrounding it and head straight for the orbital platform and eliminate it at close range, by ramming if necessary. Unfortunately, the moment the massive flagship began to move it would again become a target for a planetary weapons grid that could annihilate it all too easily. Finally, Chapter Master Gage’s alternative plan was considered: reroute all of the crippled warship's available power into its teleporter system and teleport a Kill-squad of Ultramarines, possibly two if the power lasted, directly to the Zesun Verid Yard to take control of the orbital platform from within. Led by Roboute Guilliman himself, the first Kill-squad of 50 Ultramarines assembled at the flagship’s teleportation terminal. The transverse assembly deck of the Zetsun Verid Fleet Yard was their chosen teleport destination. It was the largest interior space on the orbital platform, and thus made it less likely that imprecision in the teleport would lead to a large number of deaths. Their assault target was the Zetsun Verid Yard’s master control room, two decks up from the transverse assembly deck. Given the risk factor and the atrocious error margins of teleporter arrays, the teleport was considered a success. 46 members of the Kill-squad appeared with Guilliman on the transverse assembly deck of Zetsun Verid Yard. Only 4 Battle-Brothers had been lost during the transition and failed to rematerialise.

The Kill-squad immediately encountered heavy resistance from the Word Bearers, but with Roboute Guilliman at the fore, the enemy forces balked before the sheer killing power of an enraged son of the Emperor. With righteous fury the Primarch killed over a dozen Word Bearers single-handedly, including the daemon-possessed Astartes of the Word Bearers' vaunted Gal Vorbak. Though 8 Ultramarines were lost in the assault, the Kill-squad managed to break through into the master control room. A heavy barrage of Bolter fire greeted them as the real fight awaited. Within was the XVII Legion's First Captain Kor Phaeron, Black Cardinal and Master of the Faith, who ordered his men forward. Then he flew at Guilliman, summoning forth blasphemous energies from the Warp to assault the Primarch. Guilliman roared his challenge and charged.

Final Assault
Simultaneously on the beleaguered surface of Calth, the 4th Company struck along the Ketar Transit, a main access route that linked the container storage area to the northern facilities of the port at Lanshear. Their target was the data-engine of the Cargo Handling Guild located in the bunker system below the majestic edifice of the guildhall. Ventanus lead the foot advance across the broken streets himself, behind a column of Land Raiders. Word Bearers Astartes rose up to block the Loyalists' advance. Instead of overwhelming the Loyalist forces with well-directed firepower which might have broken or turned Ventanus’s charge, the Word Bearers simply waited for the inevitable clash of close combat. They relished the prospect of testing their Chaos-corrupted blades against the vaunted Ultramarines in hand-to-hand combat. The Traitors wanted to prove themselves against the Astartes who had been held up to them so many times as the models of what it meant to be Space Marines. The charging cobalt-blue warriors of the Ultramarines met the solid crimson line of the Word Bearers with a crash. The fighting proved both brutal and unforgiving with neither side asking for, nor receiving, any kind of quarter.

Reaching the guildhall, Ventanus lept the barricades, leading the assault forward. He tore into the fleeing Chaos Cultists who shrieked in fear at the prospect of facing the fierce Astartes officer. As the Ultramarines moved forward into the building they continued to be pounded by the XVII Legion's artillery and heavy weapons. Reaching the Cogitator that was their goal, Magos Tawren attempted to connect into it and upload the kill code that would shut down the planetary defence grid. As the Magos concentrated on gaining access to Calth's planetary defence system, the Loyalists fate took a turn for the worst. The forces of the XVII Legion's Commander Hol Beloth descended upon the guildhall with a vengeance. He had come to punish the hated Ultramarines with a force of Traitor Titans, the Word Bearers' Terminator-armoured Cataphractii and the dreaded daemon-possessed Astartes of the Gal Vorbak.

The situation became grim for the Loyalists. Traitor Titans continued to fire upon the guildhall until its very foundations began to shake from the constant pounding. Ventanus checked with Tarwen for a status update, but progress was slow as the Magos attempted to establish her cybernetic neural connection with the server. The situation outside was rapidly deteriorating as armoured columns in service to the Word Bearers moved into position and began firing at the Loyalists with a hail of shells and laser fire. Two Traitor Reaver-class Titans approached at full stride, intent on annihilating the servants of the Emperor. With the majority of his allied commanders already dead, Ventanus knew that his company line was all but broken. The Ultramarines of the 4th Company had done all that they could do, for they could not stand against the overwhelming strength of Hol Beloth’s offensive. The weapons fire intensified as the Titans continued to pound the guildhall. Tawren finally managed to upload the kill code into Calth's network so that she could prepare to purge the planetary data system and wrest control back of the defence grid. But until that control had been wrested away from the enemy, there was nothing that she could do to aid the shrinking force of Loyalists.

As the Titans moved closer to the guildhall, the last Shadowsword accompanying the 4th Company fired and damaged one of the striding giants, but the pair of Titans returned fire in unison, destroying the super-heavy tank in a vast conflagration of Titan-grade weapons fire. Ventanus knew then that their attempt had all been for naught. The Traitors of the XVII Legion had seemingly won the Battle of Calth.

Back From the Brink
High above Calth, in the master control room of the Zetsun Verid Fleet Yard, Roboute Guilliman confronted Kor Phaeron. Instead of fighting the Primarch in honourable combat, the Word Bearers' First Captain resorted to using the sorcerous abilities granted him by the Chaos Gods, slamming the Primarch of the XIII Legion against the chamber wall with a column of wretched, living darkness that burst forth from the palm of his right hand. Though shaken, the Primarch rose to his feet to face his attacker. Guilliman charged once more but was slammed back into a bulkhead by another powerful dark beam of eldritch power. Guilliman attempted to stand, staggered, then fell. The force of the Black Cardinal’s blow had been so vicious that the Ceramite of the Primarch’s breastplate was cracked. Guilliman coughed and blood dripped from his mouth. He attempted to stand once more.

Kor Phaeron blasted him once more, this time with a strange negative electrical charge that caused Guilliman to seize as if he had been caught in a violent fit. The Primarch was left on his hands and knees, his whole form smouldering, his head bowed. The Word Bearers' Black Cardinal drew a Chaotic dagger known as an Athame and stepped forward, the Primarch’s fate in his hands. He has the opportunity to end the life of the great Primarch or, with his own hand, he can turn him to the Warmaster’s cause. Just as the Word Bearers' First Chaplain Erebus had turned the Warmaster to the service of the Dark Gods on Davin's moon, so too, would Kor Phaeron achieve the impossible and bring another Primarch over to the service of Chaos. One cut of the Chaos-corrupted blade would damage Guilliman’s sanity whilst Kor Phaeron took advantage of his weakened state, and slowly sliced away the inhibitions that kept him loyal to the False Emperor. It would be a fitting revenge for the Ultramarines' actions on Khur to return to the court of Lorgar and Horus with Roboute Guilliman as a willing and pliant ally in their Great Betrayal.

The vile Word Bearer stepped forth beside the wounded Primarch and put the blade of the Athame to Guilliman’s throat. The Primarch grunted through clenched teeth as the foul blade bit. Kor Phaeron attempted to cajole Guilliman into giving up and joining the blessed cause of the Dark Gods against the False Emperor. Guilliman only muttered in reply. With every single word an effort, the Primarch explained to the Black Cardinal that he had made a grave error, for he had chosen to toy with a Primarch rather than kill him. The arrogant Word Bearer merely smiled, confident in his inevitable triumph. Guilliman made Kor Phaeron pay for his hubris. Though its energy field had long since shorted out and failed, Guilliman buried his armoured Power Fist in the Black Cardinal’s chest and ripped out one of his black, beating hearts. As one of the Ultramarines sergeants rushed to the wounded Primarch’s aid, Guilliman ordered the sergeant to forget about him for the moment and to kill the planetary data network. Unable to figure out how to shut down the data-engines, Guilliman ordered the sergeant to shoot it. But the sergeant was out of ammunition, and so he unleashed his Power Sword instead in a shower of sparks and electrical fire.

On Calth’s surface, the remnants of the 4th Company prepared to face their doom, defiant to the very end. The bunker 300 metres below the guildhall trembled as it was continuously struck by devastating salvos of enemy fire. Suddenly, Magos Tawren informed her Ultramarines allies that their circumstances had drastically changed – two Loyalist Titans had vectored into the fight. One of them had already made a kill against one of the Word Traitor Titans. Reinforcements had arrived to supplement the beleaguered 4th Company. The reinforcements exploded into the Lanshear Belt from the east, fast and mobile. One of the four governing Ultramarines Tetrarchs of Ultramar, Eikos Lamiad, the Primarch’s Champion, lead a ragged host of Imperial soldiery collected from the desert and the burning hills around the Holophusikon.

The reinforcements included a column of Land Raiders and other armour supported by three Titans: two Reavers and a massive Warlord-class Titan. An infantry force followed, moving rapidly. This force included mostly Ultramarines and Mechanicus Skitarii elements from Barrtor and the Sharud muster, but there were 20,000 Imperial Army troops as well, bringing lighter armoured vehicles and support weapons to bear. The relief force formed two prongs of assault. One was a Legion force led by a sergeant of the 112th Company named Anchise, and a captain of the 19th Company called Aethon. The other was predominantly composed of Imperial Army troops commanded by a Colonel Bartol of the 41st Neride Regiment, but it was now under the direct command of Eikos Lamiad and a lumbering Ultramarines Contemptor Pattern Dreadnought.

Hol Beloth’s Traitor forces flinched at the unbridled force of Loyalists' coordinated assault. Beloth had not believed that the shattered survivors of the XIII Legion would be able to organise a concerted response with such precision and effect. The assembled survivors of the Calth Atrocity soon expressed their fury and their vengeance against the Traitors of the XVII Legion, inflicting massive damage upon the enemy. Despite their efforts, Tawren still lacked the necessary control over the planetary data network to unleash the kill code.

Without warning, the planetary network was suddenly released from the Word Bearers' control by the events unfolding simultaneously in orbit and the swift strike of a Power Sword. The Magos received the message: Control Suspended (engine failure). She did not hesitate to upload the kill code directly into the planetary data system which quickly burned through the corrupted numerics of the Octed scrapcode and allowed her to take full control of Calth's planetary defence grid. She then ordered the orbital defence platforms to launch a devastating weapon strike upon the Traitor forces at Lanshear. Server Tawren informed all the Loyalists surrounding the guildhall to brace for impact. A column of deadly vertical light then struck the city-zone around the guildhall in the northern depot area of Lanshear as the orbital platforms unleashed a tactical nuclear strike against the Traitors. It reduced Traitor Titans, armoured vehicles, Chaos Cult auxiliary and Word Bearers infantry formations to so much ash within only seconds. However, the weapon strike occurred less than half a kilometre from where the Loyalists had taken shelter from the coming impact. Yet, the Ultramarines and Imperial Army forces remained untouched, though some eardrums had burst and many Imperial Army troops suffered radiation burns to any exposed skin. Hot ash plastered the rain-soaked Power Armour of the Space Marines, spattering them with the remnants of their enemies. Ironically, the Ultramarines' ash-covered armour soon appeared a gun-metal grey, the old livery of the Loyalist XVII Legion.

Magos Tawren next redeployed the defence grid elements available to her, hitting other surface targets. Simultaneously, she re-tasked the orbital platforms and began to systematically exact punishment upon the Word Bearers' fleet. It was now the crimson-hulled warships of the XVII Legion that were annihilated one by one. The dynamic of the entire battle had finally shifted in the Ultramarines' favour.

Pyrrhic Victory
High above, in orbit above Calth, Roboute Guilliman and his Kill-squad attempted to extract themselves from the burning master control room of the Zetsun Verid Fleet Yard. Flames and smoke rapidly filled the orbital habitats of the yard. The remainder of the Kill-squad retreated, packed tightly around their wounded, limping Primarch. As they awaited transport from the inbound Ultramarines flagship, the Primarch looked out one of the orbital platform's viewports and was stricken by what he observed. Their victory had come just in time for them to stare ultimate defeat in the face. He and his Astartes saw that while they had controlled the planetary defence grid, the Word Bearers had turned its potent weapons upon the Veridian System's sun, destabilising it. The stricken star cast a baleful shadow as it prepared to go supernova, unleashing a cataclysmic stellar explosion that would wipe out all life upon the surface of Calth.

Suddenly the Primarch’s attention was diverted when he looked below their position and saw half a dozen surviving Word Bearers carrying the bloody carcass of Kor Phaeron. Somehow, the wretched First Captain of the XVII Legion remained alive despite the fact that Guilliman had torn out his primary heart. Drawing their Bolters, the Ultramarines fired upon the retreating Word Bearers, just as their forms shimmered and vanished in a cascade of teleporter energy.

Guilliman contacted Chapter Master Gage aboard the Macragge’s Honour, and ordered him to hunt down the Infidus Imperator at all costs. He did not want Kor Phaeron to escape his ultimate fate to plague the Imperium once more. Though worried about his Primarch’s well-being, Guilliman informed Gage that they would secure one of the Ultramarines vessels docked at the Zetsun Verid Yard. The Word Bearers Grand Cruiser Infidus Imperator turned slowly in the debris-rich field of Calthian nearspace, as the wreckage of countless starships lay dying in flames behind it. It engaged its main drive and began a long, hard burn towards the outer reaches of the Veridian System. As it accelerated away, the Macragge’s Honour turned in pursuit, beginning one of the most infamous naval duels in Imperial history.

Recruitment of Rubio
While the remnants of the Ultramarines and the Imperial Army were making their exodus to the caverns beneath Calth to escape the irradiation of the world's surface, the Ultramarines' 21st Company, under the command of Captain Erikon Gaius, were dug in at one of the railway tunnels leading to Numinus City. In the midst of their defence from the attacking Word Bearers, Nathaniel Garro, the former Battle-Captain of the Death Guard, arrived on a secret mission given to him by Malcador the Sigillite, the Regent of Terra, to recruit Tylos Rubio, a former Codicier of the XIII Legion. At first, Rubio refused to abandon his brothers, but was left with no choice when in the face of the Word Bearers' overwhelming assault, he unleashed his dormant psyker abilities, saving his company but making him an outcast for disobeying the dictates of the Council of Nikaea that had disbanded the Space Marine Legions' Librariums. Rubio, with little choice, then left Calth behind with Garro in his Stormbird. Rubio would become one of seven Astartes drawn from both the Traitor and Loyalist Legions who would form the founding core of what became the secret Grey Knights Chapter of Space Marines.

Furious Abyss
At the same time that Lorgar had concentrated his Legion's offensive on Calth, a separate assault by the XVII Legion on the Ultramarines' homeworld of Macragge had been prepared, using a massive prototype Battleship specially constructed by Horus' allies in the Mechanicum before the start of the Heresy known as the Furious Abyss. Had it succeeded in reaching Macragge, the Furious Abyss would likely have annihilated the planet, the remaining two Chapters of the Ultramarines Legion garrisoned there, and the stores of precious gene-seed in the XIII Legion's fortress-monastery, the Fortress of Hera. Fortunately, the Furious Abyss was intercepted in the void before it reached Macragge by an ad hoc force led by the Ultramarines Captain Lysimachus Cestus and was destroyed, though at the cost of every Ultramarines' life.

Aftermath
The Word Bearers' use of Calth's orbital defence platforms against Calth’s sun destabilised it, tore away the outer layers of its photosphere and threatened to cause it to explode as a supernova. The Veridian System's star, its colour changing from a bright yellow to an angry blue as its internal composition shifted, immediately suffered a flare trauma, and shortly after unleashed massive solar flares that irradiated Calth with lethal levels of radiation and stripped away its once dense oxygen-nitrogen atmosphere. The surface of Calth was no longer a safe environment for human habitation or any form of organic life, nor was it possible to evacuate the planet's remaining population in time. Therefore, Captain Ventanus sent out a warning over the planetary Vox and data network to all citizens, soldiers of the Imperial Army, Astartes of the XIII Legion, and any other loyal servant of the Imperium, to move with all haste to the subterranean arcology or arcology system closest to them. The arcology systems offered sufficient protection for the remaining inhabitants of Calth and their defenders to survive the wave of solar death that was about to engulf their world. This message successfully reached many millions of people who successfully took shelter within Calth's underground warrens, which had originally been built to free up more land for agricultural cultivation. It would take several Terran years for the remnants of the Ultramarines fleet to return to the Veridian System after the Loyalist survivors rode out the storm. Unfortunately, the Forces of Chaos left on the planet also fled underground as well. Vetanus vowed that the Ultramarines and their allied Loyalist forces would continue to fight until every last Traitor on Calth had been exterminated. Thus began the phase of the conflict that would be remembered as the Underworld War.

Yet, despite the Loyalists' last-minute victory and the survival of the Ultramarines Legion and its Primarch, the Forces of Chaos could consider their assault on Calth a success. The XIII Legion had been badly crippled and no longer presented a viable threat to Horus' plan to drive on Terra. Erebus had managed to complete his blasphemous ritual on Calth's surface, which summoned a Ruinstorm to the galaxy's Eastern Fringe -- a monstrous Warp Storm larger and more destructive than anything space-faring humanity had witnessed since the days of the Age of Strife. It would split the void asunder, dividing the galaxy in two and rendering vast tracts of the Imperium impassable for centuries.

The Ruinstorm would also isolate and trap those Loyalist forces caught behind it like the Ultramarines, preventing them from coordinating their efforts and supporting one another as the Traitor Legions moved towards Terra. It would even prevent them from warning each other, for a time, of the Warmaster's betrayal and the civil war that had begun to consume the Imperium. The Ruinstorm would leave Terra alone in the void, infinitely vulnerable to the approaching shadow of Horus.

Some of the warships destroyed during the Calth Atrocity would continue to circle the tortured Veridian star for over 100,000 years as frozen wrecks. They served as silent tombs for the dead, eternal funerary monuments to the greatest betrayal in human history.

The Ultramarines learned another hard lesson at Calth. The Calth Atrocity represented the Loyalist Astartes' first sustained experience with fighting the Warp entities later known as daemons in realspace. The Ultramarines realised that their decision to accept the anti-psyker dictates of the Council of Nikaea had lead to their voluntary surrender of the one weapon that might have proved most potent against the horrors of the Warp. It was almost as if the Traitors had known what was coming, and had orchestrated events so that the Imperium would voluntarily cast aside the only practical weapon it possessed against the sorceries of the Empyrean just before it was needed most. After the Ultramarines' experience at Calth, Roboute Guilliman sought to petition the Emperor for the revocation of the Edicts of Nikaea and the reinstatement of the corps of Librarians within all of the Loyalist Space Marine Legions. When the Horus Heresy ended and Roboute Guilliman initiated the Second Founding and the Reformation of the Imperium, he would get his wish, and the Imperium would once more make use of psychic powers against the Forces of Chaos despite the inherent dangers.

XIII Space Marine Legion (Ultramarines) Personnel

 * Roboute Guilliman, Primarch of the XIII Legion
 * Tauro Nicodemus, Tetrarch of Ultramar (Saramanth), Primarch’s Champion
 * Eikos Lamiad, Tetrarch of Ultramar (Konor), Primarch’s Champion
 * Justarius, Venerable Dreadnought
 * Telemechrus, Contemptor Dreadnought
 * Marius Gage, Chapter Master, 1st Chapter
 * Remus Ventanus, Captain, 4th Company
 * Kiuz Selaton, Sergeant, 4th Company
 * Lyros Sydance, Captain, 4th Company
 * Archo, Sergeant, 4th Company
 * Ankrion, Sergeant, 4th Company
 * Barkha, Sergeant, 4th Company
 * Naron Vattian, Scout, 4th Company
 * Saur Damocles, Captain, 6th Company
 * Domitian, Sergeant, 6th Company
 * Braellen, 6th Company
 * Androm, 6th Company
 * Evexian, Captain, 7th Company
 * Amant, 7th Company
 * Lorchas, Captain, 9th Company
 * Aethon, Captain, 19th Company
 * Erikon Gaius, Captain, 21st Company
 * Tylos Rubio, 21st Company
 * Honoria, Captain, 23rd Company
 * Teus Sullus, Captain, 39th Company
 * Greavus, Sergeant, 39th Company
 * Kaen Atreus, Chapter Master, 6th Chapter
 * Klord Empion, Chapter Master, 9th Chapter
 * Vared, Chapter Master, 11th Chapter
 * Ekritus, Captain, 111th Company
 * Phastorex, Captain, 112th Company
 * Anchise, Sergeant, 112th Company
 * Sharad Antoli, Chapter Master, 13th Chapter
 * Taerone, Captain, 135th Company
 * Aeonid Thiel, Sergeant, 135th Company [marked] - Thiels helmet was painted red to indicate that he was marked for censure for running theoretical scenarios for combating other Space Marines - at the time, this was to satisfy his own curiosity and a yearning to account for all possibilities. However, due to the substantial losses amongst the XIII command personnel, Thiel emerged as a proficient leader and resourceful fighter. When the time came to organise the shattered remnants of the XIII into fighting squads, the new leaders were marked with the red helmet. This pattern continues to the present day.
 * Evido Banzor, Chapter Master, 16th Chapter
 * Heutonicus, Captain, 161st Company
 * Jaer, Apothecary, 161st Company
 * Kerso, 161st Company
 * Bormarus, 161st Company
 * Zabo, 161st Company
 * Anteros, 161st Company
 * Honorius Luciel, Captain, 209th Company

Ultramarines Fleet

 * Aegis of Occluda (Severely Damaged)
 * Antipathy – Cruiser (Destroyed)
 * Antrodamicus – Grand Cruiser (Destroyed)
 * Antropheles – Troop Transport (Destroyed)
 * Burnabus – Escort (Catastrophic Damage)
 * Campanile – Fleet Tender (Destroyed)
 * Cavascor
 * Constellation of Tarmus – (Destroyed)
 * Cornucopia
 * Courage of Konor – Carrier (Destroyed)
 * Deliverance of Terra (Destroyed)
 * Gauntlet of Glory – Battle-Barge
 * Gauntlet of Victory – Battle-Barge
 * Gladius – Escort (Destroyed)
 * High Assent
 * Hope of Narmenia – Battle-Barge (Destroyed)
 * Janiverse – Frigate (Destroyed)
 * Jeriko Rex – Fast Escort (Catastrophic Damage)
 * Johanipus Artemisia – Carrier (Destroyed)
 * Lutine
 * Macragge’s Honour – Flagship of Primarch Roboute Guilliman
 * Menace of Fortis (Destroyed)
 * Mlatus
 * Mlekrus – Strike Craft
 * Remonstrance of Narthan Dume – Battleship (Destroyed)
 * Samothrace
 * Sanctity of Saramanth – Heavy Destroyer (Destroyed)
 * Solonim Woe
 * Sons of Ultramar – Battle-Barge (Destroyed)
 * Spirit of Konor – Battleship (Destroyed)
 * Stations of Ultramar – Picket Cruiser (Destroyed)
 * Steinhart – Carrier (Catastrophic Damage)
 * Suspiria Majestrix – Grand Cruiser
 * Tarmus Usurper (Catastrophic Damage)
 * Testament of Andromeda – Carrier Ship (Moderate Damage)
 * Triumph of Iax (Crippled)
 * Ultimus Mundi – Battleship
 * Valediction – Carrier (Destroyed)
 * Vernax Absolom
 * Vospherus – Carrier (Destroyed)

Adeptus Mechanicus Fleet

 * Phobos Encoder (Destroyed)

Titans

 * The Buring Cloud (Reaver-class)
 * Kaskardus Killstroke (Reaver-class)

Non-Astartes

 * Ollanius "Oll" Persson – Perpetual

Imperial Army Regiments

 * 2nd Erud Ultima
 * 6th Neride ‘Westerners’
 * Neride Regulators 10th
 * 14th Garnide Heavy Infantry
 * 41st Espandor
 * 19th Numinus
 * 21st Numinus
 * Numinus 61st, Regular Infantry

Imperial Army Personnel

 * Colonel Sparzi, commander, Neride 10th
 * Bowe Hellock, Sergeant, Numinus 61st
 * Dogent Krank, Numinus 61st
 * Bale Rane, Numinus 61st

Adeptus Mechanicus Forces

 * Kalkas Cohort of Skitarii 

Adeptus Mechanicus Personnel

 * Uhl Kehal Hesst, Server of Instrumentation
 * Meer Edv Tawren, Tech-Magos of Analyticae
 * Uldort, Tech-Magos
 * Arook Serotid, Master of Skitarii
 * Cyramica, Skitarii
 * Shipmaster Sazar, captain of Macragge’s Honour
 * Bohan Zedoff
 * Pelot, Tech-Magos
 * Shipmaster Ouon, captain of Sanctity of Saramanth
 * Hommed, Representative of Macragge’s Honour

XVII Space Marine Legion (Word Bearers)

 * Lorgar Aurelian, Primarch of the XVII Legion
 * Kor Phaeron, The Black Cardinal
 * Erebus, Dark Apostle
 * Argel Tal, Crimson Lord of the Gal Vorbak
 * Essember Zote, Battle-Brother, Gal Vorbak
 * Foedral Fell, Commander
 * Morpal Cxir, Commander
 * Hol Beloth, Commander
 * Maloq Kartho, Apostle to Hol Beloth
 * Sorot Tchure, Captain
 * Ulmor Nu

Word Bearers Fleet

 * Crown of Colchis 
 * Destiny’s Hand – Battle-Barge, flagship of Dark Apostle Erebus
 * Flame of Purity
 * Infidus Imperator – Grand Cruiser, flagship of First Captain Kor Phaeron
 * Liber Colchis
 * Spear of Sedros

Traitor Titans

 * Mortis Maxor (Destroyed)

Chaos Cults

 * The Ushmetar Kaul, “The Brotherhood of the Knife”
 * Criol Fowst, Confided Lieutenant
 * The Tzenvar Kaul, “The Recursive Family”
 * The Jeharwanate, “The Ring”
 * The Kaul Mandori, “The Gene-kin”
 * Vil Teth, Gene-named

Canon Conflict
According to the novel Know No Fear, the Battle of Calth concluded just short of twenty-four hours after the Word Bearers launched their assault, a moment the Ultramarines later called Mark Zero, while the ground forces were evacuating the surface to escape the poisoned sun's radiation; however, according to the audio book Garro: Oath of Moment, the Ultramarines' 21st Company has been dug in "for days" following the initial Word Bearers attack.