Operator Recommended Game Story · Warhammer 40,000 Part 24

Spoiler and Scope of This Episode

This episode covers the Battle of Calth during the Horus Heresy. It follows the gathering of the Ultramarines Legion, the surprise attack by the Word Bearers, the battle inside the Macragge's Honour, the counterattacks on the ground and in orbit, the corruption of the Veridian star, and the subsequent retreat underground. I have reconstructed these events in my own words after reading the official 2015 Tempest war records and the Mechanicum muster data. This account is presented as a narrative from an in-universe historian, distinguishing between established facts and what the historian assumes or admits to be unknown. This is not a direct translation of the original novel's prose or dialogue. Detailed ground combat, the personal fates of individuals, and the underground war will be covered in subsequent linked deep-dive episodes.

Table of Contents for This Episode

1. The Warning That Has Not Yet Arrived / 2. The Order Protecting the Five Hundred Worlds / 3. The Gate Opened to a Brother / 4. The Last Bell Tolled by the Campanile / 5. The Fleet That Became the Enemy / 6. Commands Severed in Sky and Earth / 7. The Logic of Machines and the Time of Men / 8. Another World Opened on the Flagship / 9. The Red Helmet and the Surviving Commander / 10. Two Paths to Find the Captain / 11. The Father Returned from the Vacuum / 12. Ventanus's Path and the Orbital Struggle / 13. The Night Returned by the Reclaimed Weapon / 14. From Under the Sun to the Underground

1. The Warning That Has Not Yet Arrived

The entire galaxy did not immediately know that betrayal had been revealed at Isstvan. The Emperor's realm was built upon the vast distances between stars. Worlds connected by treaties, people speaking different languages, and planets maintaining their own governance and production all belonged to the same Imperium. Conveying the truth of a single battlefield to all those places held a different set of difficulties than simply sending a fleet to conquer them.

Astropaths sent messages to distant minds through the Warp. The news did not arrive as clear text like a daily letter. Symbols, dreams, and signals requiring interpretation crossed the distance. Now, the turbulence of the Warp distorted even those paths. Ships carrying warnings vanished, and the stories of those who survived became rumors that could not be verified in some worlds. The storm of betrayal severed the Imperium's understanding and judgment even before the shells began to fall.

Horus needed that gap. Once Terra began to prepare, the war could not be ended by a single surprise attack to topple the Emperor. If the loyalist Legions gathered their strength, the traitors could be pressured from both front and rear. He sought to scatter the brothers who would not heed his commands and tie down their manpower and production capacity. Even after the destruction of Isstvan, there were Primarchs across the galaxy who still held their Legions intact.

Ultramar was a particularly great power among them. What Horus needed was a situation where Roboute Guilliman could not immediately move toward Terra. For Lorgar, the name was also connected to an old wound. When they knelt in the ashes of Monarchia, the Ultramarines were the Legion that executed the Emperor's command. However, explaining the war of Calth solely through vengeance misses the scale of the trap that had been laid. This was an attack that repaid the past of a city, while simultaneously trapping the future of the galaxy.

2. The Order Protecting the Five Hundred Worlds

Guilliman's strength did not lie solely in the number of warriors he led. The various worlds of Ultramar produced the weapons, supplies, people, and ships the Legion required. The movement of troops relied on the organization of administration and production. How to maintain a place after a victory and where to prepare for the next war were interconnected issues within his domain. What Horus feared was not just the warrior brother, but the foundation that brother had built.

On Calth, an expedition against the Ork forces of Ghaslakh was being prepared. The order came with the authority of the Warmaster. The plan was for the Ultramarines and the Word Bearers to gather together and attack the same enemy. It took a long time to call in units scattered far and wide. There were troops that had to withdraw from current battles, and fleets whose voyages were delayed due to the instability of the Warp. Although the plan had flaws from the path toward Calth, it was difficult at the time to interpret all those delays as signs of betrayal.

The planet was transformed to accommodate the Legion. Camps, training grounds, ammunition depots, and maintenance facilities were built. Residents and laborers were mobilized for the preparations. Ships hung in shipyards and orbital stations, and maintenance crews continued their final work inside ships with lowered reactors ahead of the battle. The defensive network was densely configured to detect and block incoming enemies. It was a place too strong to be breached head-on from the outside.

It was not only the Ultramarines who gathered there. There were Imperial Army units made of ordinary humans, Solar Auxilia prepared for vacuum combat, Tech-Priests and combat automata, and the war machines of Titan Legions and Knight Houses. The equipment, faith, and command structures they carried were all different. Yet, they were preparing under the name of the same expedition. The planet's massive military power was completed when various groups were connected, and it was precisely that connection that, if destroyed, could cause the power of each group to fall apart separately.

3. The Gate Opened to a Brother

The Word Bearers did not enter as enemies. Calth's gates and defensive systems allowed the allied forces to pass. Guilliman placed his warriors close to those of his brother Legion. There was an expectation in that placement that those who would stay and fight together could reduce the distance of the past. Until the war began, the most dangerous enemy to the defenders was already inside their own camp.

Erebus and the units following him arrived first, followed by transport ships and warships that filled the gathering space. The strength of the Word Bearers is estimated in official records as an approximate number. This is because it is difficult to accurately reconstruct every unit due to false signals and disguised affiliations. If one hides this uncertainty and counts only the numbers firmly, one ends up claiming to know more facts than the historians of the future actually do. What is clear is that a large-scale attack force entered under the guise of an ally.

They observed the preparing Ultramarines. They could see which units stayed where, how the officers moved, and in which facilities supplies were piled. Although some found the repetition of strange rituals and training odd, that suspicion did not lead to sufficient investigation amidst the command's judgment to restore the relationship between the two Legions. The information the attack force would use was not stolen through long reconnaissance, but gathered within the trust granted to a brother.

The records of 'Tempest' do not conclude that the entire surface began its attack at one precise moment from the start. In remote encampments, there remained traces that the slaughter began before the orbital catastrophe. In other places, the attack proceeded in earnest only after the commanders were eliminated. This difference shows how meticulous the betrayal was, and at the same time, how hatred, religious obsession, and incomplete communication were mixed within it. It was not a war where everyone moved in the same way when a single signal sounded.

4. The Last Bell Tolled by the Campanile

The old fleet support ship Campanile went silent in outer space. Official war records do not confirm until the end who seized the ship or in what manner. There only remain traces of the possibility of intervention by Warp-tainted entities and a seizure predicated on sacrifice. The ship was no longer a vessel for delivering supplies, but had become a weapon to be flown toward the gathered fleet.

After a sudden acceleration, the Campanile pierced through the ships. The collision did not stop at breaking other ships. Its mass and speed turned into a disaster for the dense orbital facilities. The remaining high-density reactor section struck the Calth Veridian Anchor. The ships connected to the station, the crews within them, and the facilities responsible for the planet's defense vanished together.

The ripples of the explosion pushed away the next disaster. The unmoored battleship Antrodamicus fell toward the planet. The massive hull, descending over the capital Numinus, destroyed the Kalkas watchtower. It was a crucial location for the Mechanicum's command and control. The centers established in orbit and on the ground disappeared almost together. People watching the ship falling from the sky could not even fully understand that the sight before their eyes was the beginning of a war.

Communication equipment was shut down and detectors collapsed. Under normal circumstances, reserve systems could have taken over control, but the corrupted code planted by the Traitor Mechanicum prevented even that recovery. Some officers were alive but could not convey messages to other captains. A few open channels were flooded with requests for help and voices seeking orders. A well-organized defense system required not only powerful weapons but also the information to connect them for use. What was severed first on Calth was that very connection. Even if battleships remained, the fleet, unable to understand each other's situations, was divided into small, isolated worlds in an instant.

5. The Fleet That Became the Enemy

They did not accept this damage merely as an accidental incident, but they did not immediately understand the betrayal of their brothers either. For some commanders, the most familiar interpretation was a preemptive strike by the Orks, the target of the expedition. Even when the Word Bearers' bombardment began, there were those who asked if they were unable to distinguish between friend and foe in the chaos. The relationships built while fighting wars together required time to accept the facts before their eyes.

The red ships did not wait for such time. The attack force rushed in with reactors, shields, and armaments prepared. Some of the Ultramarines' ships had not even awakened from the low output required for maintenance. Ships in different states of readiness were on the same battlefield, but only the side that intended to create that battlefield from the start knew what would happen. The firepower of numerous stations and ships could not bridge that difference in information.

Guilliman contacted Lorgar. Guilliman demanded that his brother stop the attack, but Lorgar refused and cut the connection. That reaction was not an explanation to clear up a misunderstanding. From that moment on, the Ultramarines' response was no longer about handling an accident. Guilliman had to authorize his warriors to defend and counterattack. The order to use weapons against the Legion that had fought alongside them in the Great Crusade was issued to the Calth fleet.

Kor Phaeron headed toward another important location. A group of Word Bearers who had entered the Ultramarines cruiser Samothrace seized the ship. The ship, changed with the help of the Traitor Mechanicum, moved to the Zetsun Verid Yard. This station becomes the link that binds the fate of the fleet and the planet in the subsequent war. What the attack force needed was not only to destroy all the enemy's ships, but to take the handles of the defense the enemy had built into their own hands.

6. Commands Severed in Sky and Earth

On the ground, recognizing the faces of the same Legion did not guarantee survival. Troops who shared camps, maintenance facilities, and training grounds turned and attacked. Ultramarines units that lost their commanders had to find weapons before they could even ask about the situation. The already collapsed communication network and falling debris blocked the paths for troops to gather. Cities and residents not involved in the fighting were also within the scope of the destruction.

Orbital bombardment was not focused only on military facilities. Cities burned, and energy weapons poured into the sea pushed away hot steam. The terrain and air of the living planet changed together. To those who had been preparing for an expedition under the sun, Calth now became a place where death fell from the sky. Because the attack force had seized the defensive network, those weapons returned to the surface they were supposed to protect instead of stopping the invaders.

The Word Bearers' objectives overlapped with strategic destruction and religious ritual. Records cannot fully explain why they chose specific sacrifices and killings. However, survivors left accounts that as the massacre grew, strange weather, auroras, and the appearance of Warp entities increased. It was a process where the death of the planet wounded even the Warp. People began to encounter in the real world entities that were difficult to explain with the enemy tactics they had known.

Ordinary soldiers also fought in the gaps. The Solar Auxilia of the orbital facilities guarded the corridors based on their sealed combat suits and training. Although they were physically and technologically inferior to Space Marines, the defenders who knew the paths for retreat could create time to reach the next defensive line. The resistance of some stations met an end where they could not survive themselves while blocking the attack force. Such victories also remain in the records of Calth.

7. The Logic of Machines and the Time of Men

Not all defense forces were paralyzed in the same way. The Taghmata led by Cronn Barbarel gathered various Tech-priest groups of Ultramar. Each group's research, weaponry, and communication methods were different, and because of that, they were not completely integrated with the planet's central network. Even after losing the central facilities, there were forces that could use separate encrypted communications. The difference that would have been inconvenient in normal times became a brief possibility for survival in the disaster.

Barbarel's judgment turned to the fact that the attacking force was trying to destroy him and his allies. The fact that they originally came with different banners did not change that reality. The Mechanicum forces fought a delaying action, calculating time and losses. While the armored units of Anuaris held the attack force, the Ultramarines could retreat. At Erud, they drew the enemy's charge into the rocks and terrain to create a gap for other troops to gather.

Sylator's Magenim automata responded to hostile actions according to their built-in logic after control was lost. It did not require hesitation to understand the betrayal. However, that automatic response did not mean perfect discernment. Official gathering data records that loyalist forces who fired at them in the chaos were also attacked. The speed of the machine's reaction and the problem of having to distinguish allies among people were different issues.

The Mechanicum on Calth was not a single unified army from the beginning. Barbarel's group, the Xerxes forces from Accatran, and Titans and other organizations with separate authority were together. There was also an old feud between Konor and Anuaris. One must know those differences to understand who opened which path and where control was lost. The problems of technology and loyalty that began with the schism of Mars had now arrived on the battlefield of Ultramar.

8. Another World Opened on the Flagship

On the Macragge's Honour, Guilliman faced Lorgar again. The brother who appeared through communication revealed that the attack on Calth was not an isolated action but part of a larger betrayal. The results of Isstvan he conveyed also contained claims that contradicted the facts. Official war records distinguish that those claims were what Lorgar said at the time, and that they do not match the facts revealed later. Readers also do not need to read everything the villain says as the confirmed truth of the world immediately.

However, even if communication was cut, the figures before their eyes did not disappear. What appeared as holograms had turned into bizarre entities with physical bodies. The explosion they caused tore the bridge. Guilliman and several officers were swept into space, and Shipmaster Zedoff, who was in command of the ship, died. To the war attacking the fleet from the outside, another attack that broke the rules of reality from the inside was added.

Warp entities appeared inside the ship. The procedures for sealing off compartments or venting gas to stop intruders were prepared against life forms they already knew. This enemy did not stop with such responses. Crews and warriors experienced situations where their familiar equipment did not produce the expected effects. The survivors had to learn the nature of the enemy, which the Imperium had not taught them, directly on the battlefield.

Marius Gage survived the destruction of the bridge. Even though he pulled out wounded comrades, he could not save all those lives. Attacks awaited the survivors who went down below. In a situation where the flagship could not be moved as a single army, small groups found places to hold out on their own. The first task of protecting the ship was not to form a formation for victory, but to leave a living connection until the next officer and the next squad arrived.

9. The Red Helmet and the Surviving Commander

Aeonid Thiel's helmet was red. The red color of this period was not a mark of honor from the beginning. He was a warrior awaiting censure. His tendency to think outside accepted norms had called him before his commander, but that very thinking became necessary when dealing with enemies encountered for the first time inside the ship. The war overturned the criteria for judging people and the criteria for saving people in the same place.

Where Thiel was, there were weapons collected by Guilliman. He fought with special swords and axes he could reach, and confirmed that those weapons were more effective against the enemy than bolt pistols. He had to ask what the enemy was and choose methods that actually worked. That judgment does not mean he fully understood all the mysteries from the start. Calling the enemy demons and noticing the effectiveness of melee weapons and proving all the principles are distinct stages.

The scattered forces gathered following Thiel. Not only Ultramarines, but also armed personnel within the ship, Solar Auxilia, and other crew members were in that group. Klord Empion of the 9th Chapter and Heutonicus of the 161st Company also led survivors from other places. Actions such as protecting recruits, advancing to other decks, and reviving small contacts began to meet within the massive flagship.

Gage was seriously wounded and lost an arm. After Thiel saved him, the officer acknowledged that the subordinate's method was effective. In a situation where it was difficult to lead the battle directly due to his injuries, he handed over command. The hierarchy remained, but the purpose of maintaining it also remained. For the Ultramarines to become an army that could fight again, they had to listen to those who had the judgment necessary to survive now.

10. Two Paths to Find the Captain

The fact that the auxiliary bridge of the flagship remained did not mean the ship could be moved immediately. A shipmaster who knew the ship was needed, and a Tech-priest to regain control of the machines was also needed. Gage pinned his hopes on the fact that Shipmaster Hommed was among the survivors of other ships rescued earlier. It was not even confirmed if he was still alive. The fate of the massive battleship was connected to one person trying to survive in a storage facility in the corner.

Thiel went to find Hommed. When the enemy swarmed toward the work platform where the survivors were, the purpose of the bullets he fired was not just to kill all the enemies. If he could divert their attention and gain time, he could move the people down. While the platform moved, the warriors retreated and held the enemy back. When the bulkhead finally closed, the captain, who was the reason they endured the attack, was alive.

Gage's troops arrived at the machine sanctuary where the Tech-Priests were holding out. Even warriors who followed the secular truths of the Imperium could respect the sacred places of their allies. However, the battle was continuing inside the door, and the technicians there could not stop the enemies surrounding them. The Legion opened the door and entered, utilizing the effectiveness of the melee combat they had learned from Thiel. The priests' ritual weapons and automata were also added to the fight.

As the shipmaster, Tech-priest, and Legion commander gathered again at the auxiliary bridge, the Macragge's Honour regained control. The recovering detection and communication simultaneously returned horrific information. The fleet was greatly reduced and the Legion on the ground was being destroyed. The whereabouts of the Primarch were still unknown. Still, being able to know the situation meant being able to design a counterattack. The surviving ship finally began to exchange words again with the survivors on the planet's surface.

11. The Father Returned from the Vacuum

Outside, the Word Bearers' ships were clinging to the flagship. If the infiltration devices and connection corridors were not severed, the recovery inside could collapse again. Empion sent troops outside the hull. Warriors equipped with armor and vacuum gear moved over the hull, which was as large as a city. While listening to their own breath and communications, a silent battle unfolded before their eyes.

Guilliman appeared where Thiel's squad was being driven back. The Primarch who had disappeared when the bridge collapsed had returned alive. The records of 'Tempest' leave the fact that he endured the vacuum without a helmet as something that even later researchers could not fully explain. It does not erase that incomprehensibility and explain it as an ability possible for everyone. The important fact for the warriors that day was that their father had returned and saved them.

Guilliman attacked the Word Bearers. A ferocity different from his usual cold command was revealed in that fight, but for the sake of the remaining war, he had to return inside the ship. His sons led him in. Now the Legion had a living Primarch and a controllable flagship. That did not return everything they had lost. However, the despair that total destruction was already determined could be broken.

The counterattack at Calth was not achieved solely by the return of that one person. Thiel gathered survivors, Gage took over command, and Hommed and the technicians moved the ship again. Because there was a path they had left, the Primarch's power could also be connected back to the Legion's command. The abilities of a great figure and the work of people holding out in small places did not replace each other. They were forces of different sizes turning back the same war.

12. Ventanus's Path and the Orbital Struggle

On the ground, Remus Ventanus was also gathering scattered resistance. The captain of the 4th Company tied together the surviving troops and headed toward the path to reclaim Calth's defensive network. What was important was not to find the answer only in reducing the number of enemies one by one. If they left the weapons seized by the Word Bearers as they were, the surviving troops would have to keep fighting the destruction falling from the sky. They had to reclaim the place that connected the power of the war.

Under the Lanshear Guildhall, there remained data devices not tainted by the enemy's corrupted code. The loyalist Mechanicum could purify the system there. However, waking the devices on the ground was not enough. The enemy still held superior control at the Zetsun Verid Yard in orbit. Both places had to be cleared together for the defense network to regain its function. It was a counterattack where they had to wait for each other's success on the ground and in space.

Ventanus's attack included Land Raiders, heavy armored vehicles, warriors gathered from other units, Skitarii, and regular soldiers. Even after destroying the Titans in the path and reaching the Guildhall, the battle did not end. Enemy forces swarmed again. The remnant company led by Sergeant Anchise, Aethon's forces, Eikos Lamiad, and Dreadnought Telemechrus added to the resistance. Those who survived on their respective paths gathered for the fate of the same building.

Guilliman headed directly to the orbital station. The flagship's teleportation was a great burden that could not easily guarantee a return path. If he failed, he could lose both the attack force and the Primarch. However, they could not guarantee the Legion's survival while leaving the defensive network to the enemy. It was not a situation where a commander calculating risks could make a risk-free choice. He attacked the other end of the path that the ground troops were opening.

13. The Night Returned by the Reclaimed Weapon

Kor Phaeron brought Guilliman to his knees with the power of the Warp. An opportunity arose to kill the Primarch in the station's control room, but instead of finishing him immediately, he made an offer to stand on their side. That choice left time for Guilliman to gather his strength. The Ultramarines' Primarch pierced the traitor's armor and tore out his heart. However, that wound did not mean the final death of Kor Phaeron. Behind the official records, he who returned alive even after losing his heart appears again.

The station finally fell into the hands of the loyalists. As the Tech-Priests under the Guildhall restored the connection, the defensive network that had been pressuring the Legion turned toward the attack force. The enemy that had been besieging them on the ground collapsed under the orbital fire. The power the warships had been enduring was no longer safe either. To the space where the wreckage of the Ultramarines had been floating, the wreckage of the Word Bearers was added. The moment the weapons they had seized and used turned back, the traitors lost the initiative of the battlefield.

Kor Phaeron was moved to the Infidus Imperator but was too injured to command. The captain tried to evacuate the remaining ships. Guilliman had Gage lead the Macragge's Honour to pursue them. The conclusion of this pursuit and the separate voyage across the Warp are stories to be continued, distinct from the ground battle of Calth. The official war records of that day also divide the path here, stating that a different story is needed.

The Ultramarines survived. But saying they survived did not erase the cost. The large number of dead and wounded left in the war records and the reduced fleet made it difficult to send forces outside of Ultramar immediately. The victory of driving away the enemy before their eyes and the ability to act in the entire war were not the same thing. They had prevented the annihilation of the Legion that Horus had aimed for, but it was after losing too much strength to prevent the next wound Ultramar would receive.

14. From Under the Sun to the Underground

Calth became a planet where the victors could not return and resume their lives. The weapons the attack force turned toward the star changed the light of Veridia into something lethal. Even if they reclaimed the defensive network, the changes that had already occurred could not be reversed. The air on the surface and the radiation environment could not protect people. Now, to survive, they had to go down from a life of looking at the sky.

Ventanus and the remaining troops led the residents to the underground arcologies. Troops who could endure radiation a little longer with their armor and modified bodies took on the time for those without such protection to move. It was an evacuation that required leaving behind the homes of their childhood, familiar streets, and workplaces under the light. The massive underground facilities were a possibility for a new life and also the entrance to the next battlefield. The war against the remaining Word Bearers continued there as well.

The fleet also had little time for choices. Several worlds in Ultramar were asking for help, and the Warp storms could soon close the paths of navigation. Guilliman had to load the troops and weapons he could carry first to respond to a larger war. There was no transport capacity left to save all the residents and all the warriors at the same time. Among those who remained, there were warriors who chose to stay to protect the residents, and there were those who could not leave due to the situation. The same remaining cannot be explained only as a single glory or a single compulsion.

Barbarel's Tech-priests and forces also resisted to the end and covered the retreat. Their public gathering records state that a small number of survivors remained, and that no forces were confirmed to have survived until the end of the Underworld War. That leaves the fact that the names of priests and the number of machines cannot show all the time that saved people. The survival of Calth was not just a story of the Emperor's sons. There were also those within it who left the next day to others with their final operations.

Guilliman left, leaving behind the intention to return to Calth. A long war began underground, and the wounds on the surface and in orbit remained for generations to come. The distance between warriors who had experienced the betrayal and those who had not did not disappear easily. Calth became a memory that made one ask again what could be trusted, beyond the names of defeat or victory added to the Legion's ledger.

The next main episode heads to other worlds of Ultramar. The Shadow Crusade of Lorgar and Angron, Armatura and Nuceria, and the Ruinstorm growing larger are connected to Calth. Even after the resistance of Calth saved the Legion, the war of the Five Hundred Worlds did not end. The commander who promised a future to return to had to head to another battlefield to leave that future behind.

Reading Names and Concepts Together

Calth: The muster planet of Ultramar. After the betrayal of the Heresy and the stellar corruption, life on the surface collapsed and the underground war continued.

Veridia: The name connected to the star system to which Calth belongs and its star. After the attack using the orbital defense network, the star's lethal light forced an evacuation.

Campanile: The seized fleet support ship. Its collision attack began the destruction of the Calth Veridian Anchor and the muster fleet. War records do not confirm the specific entity behind the seizure.

Macragge's Honour: The Ultramarines flagship. After the destruction of the bridge and the intrusion of Warp entities, the survivors recovered the auxiliary bridge.

Aeonid Thiel: An Ultramarines Sergeant who was wearing the red helmet of disciplinary action. He observed the nature of the enemy and found effective combat methods, contributing to the flagship's counterattack.

Marius Gage: The commander of the Ultramarines 1st Chapter. Amidst his injuries, he organized the recovery of the flagship's command and later took charge of pursuing the enemy flagship.

Remus Ventanus: The 4th Company Captain who led the ground resistance of Calth. He connected the data device of the Guildhall with orbital operations and took charge of the evacuation.

Cronn Barbarel: A Tech-Priest who commanded various Taghmata gathered for the Calth expedition. It is not expanded that all Titan Legions and separate organizations were under his identical command.

Zetsun Verid Yard: The orbital station seized by Kor Phaeron. It becomes key to reclaiming the defensive network along with the device at the ground Guildhall.

Underworld War: The long-term fight that continued in massive underground facilities after the surface of Calth became dangerous. It distinguishes the first day of betrayal and the various stories thereafter.

Already Published Linked Episodes

Warhammer 40,000 Part 14 — Monarchia and Lorgar's Conversion: /en/community/tm-warhammer40000-saga-part014-20261008

Warhammer 40,000 Part 22 — Isstvan V and the Drop Site Massacre: /en/community/tm-warhammer40000-saga-part022-20261008

Warhammer 40,000 Part 23 — The Schism of Mars and the Mechanicum Civil War: /en/community/tm-warhammer40000-saga-part023-20261008

Official Materials and Verification Scope

Verification Date: October 8, 2026 (UTC). This was written based on the settings and events confirmed in the official public materials below. Public samples and setting introductions only verified the relevant public scope and do not indicate that the entire book was read. The illustrations are AI reconstructions, not official images.

The War at Calth — official public Tempest extract

The Mechanicum Taghmata of the Calth Muster — official public extract

Fight the underground war with new Horus Heresy missions and an Ultramarines unit

The Ultramarines Legion Praetors are ideal for avenging Calth

Traitor Legiones Astartes official painting guide — Word Bearers