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At a glance
Calth is one of the most important worlds of Ultramar, the realm of the Ultramarines. Before the Horus Heresy, it was a jewel of the realm, a world of shipyards, cities and gardens where the XIII Legion mustered for its campaigns. Its orbital docks were among the greatest in the galaxy, and its people prospered under the rule of Roboute Guilliman.
All of that changed at the Battle of Calth. The Word Bearers, under orders from Lorgar and Horus, launched a surprise attack on the Ultramarines, who had gathered at Calth to prepare for a campaign against the Orks. The Word Bearers betrayed their brothers, and the battle that followed was one of the most savage of the Heresy. In the course of the fighting, Calth's star, Veridian, was damaged by the Word Bearers' weapons, and its radiation turned the planet's surface into a lethal wasteland.
The people of Calth survived by retreating into vast underground caverns, where they built the arcologies that are their homes to this day. The Ultramarines fought on in the darkness, hunting the Word Bearers through the tunnels in what became known as the Underworld War. Calth endured, but it has never recovered its former glory. It remains a symbol of the Ultramarines' resilience, and a reminder of the Word Bearers' treachery.
Geography & environment
Calth was once a beautiful world of oceans, forests and cities. Its orbital shipyards were among the largest in the galaxy, and its surface was dotted with gardens and parks. But the damage done to its star changed everything. Veridian became unstable, and its radiation stripped the planet's atmosphere of its protective layers. The surface is now a sterile wasteland, scorched by the sun and swept by radiation.
Beneath the surface lie vast networks of caves and caverns, which the people of Calth have turned into cities. These arcologies are home to millions of people, and they are self-sufficient, with their own farms, factories and governments. They are linked by tunnels and transit systems, and they are protected from the radiation above by kilometres of rock.
The surface is not entirely abandoned. Some industries and fortifications operate there under heavy shielding, and the Ultramarines maintain bases and defences. But for the most part, the surface is a dead zone, and the people of Calth live their lives underground.
The arcologies
The arcologies of Calth are among the wonders of Ultramar. They were built in the vast natural caverns beneath the planet's surface, and they have grown over the millennia into sprawling cities. Each arcology has its own character, its own industries and its own culture. Some are grand and beautiful, with soaring vaults and lights that imitate a lost sky. Others are cramped and industrial.
The arcologies are designed to be self-sufficient. Farms grow food under artificial light, water is recycled and purified, and the air is filtered and circulated. The arcologies are also heavily defended, with fortifications, weapons and garrisons. The memory of the Underworld War has taught the people of Calth never to be complacent about the threats that may lurk in the depths.
The people of the arcologies are proud of their resilience. They have survived catastrophe and rebuilt their world in the darkness. They regard themselves as a people apart, shaped by their history and their environment. They are fiercely loyal to the Ultramarines, who fought and died to save them.
History
The jewel of Ultramar
Before the Heresy, Calth was one of the most important worlds of Ultramar. Its shipyards built warships for the Ultramarines and the Imperial Navy, and its people were prosperous and loyal. Guilliman regarded it as one of the jewels of his realm, and it was chosen as the muster point for the campaign against the Orks of the Ghoul Stars.
The betrayal
When the Ultramarines gathered at Calth, they were joined by the Word Bearers, who had been ordered by Horus to assist them. The Ultramarines suspected nothing. They had no reason to doubt their brothers, and they welcomed them as allies. But the Word Bearers had come to destroy them. At a signal from Kor Phaeron, the Word Bearers' fleet opened fire on the Ultramarines' ships, and their warriors attacked the Ultramarines on the ground. The Ultramarines were taken completely by surprise.
The Battle of Calth
The battle that followed was one of the most savage of the Heresy. The Ultramarines, despite their losses, fought back with fury. In orbit, the fleets clashed in a desperate battle, and the Word Bearers attacked the Veridian star with weapons that damaged it and caused deadly solar flares. On the ground, the Word Bearers' cultists and daemons unleashed horror on the planet's population. The Ultramarines suffered terrible losses, but they rallied, and with Guilliman's leadership, they drove the Word Bearers back.
The Underworld War
As the star's radiation scorched the surface, the survivors fled underground. The Word Bearers followed them, and a long and brutal war began in the darkness. The Ultramarines hunted the Word Bearers through the tunnels, fighting in the close confines of the caves. The war lasted for years, and it was one of the hardest campaigns the Ultramarines ever fought. In the end, the Word Bearers were defeated, and Calth was saved, though at terrible cost.
The Word Bearers' treachery
The Word Bearers' attack on Calth was one of the most infamous acts of the Heresy. It was a betrayal of trust, an attack on brothers who had welcomed them as allies. It was also a carefully planned operation, designed to destroy the Ultramarines before they could join the war against Horus.
The attack was led by Kor Phaeron, the Word Bearers' Master of the Faith, and it was guided by Erebus, the First Chaplain who had led the Legion into the worship of Chaos. Lorgar himself was not present, but he had sanctioned the attack, and it was part of his larger plan to weaken the loyalist Legions. The Word Bearers hoped that by destroying the Ultramarines, they could cripple the loyalist cause and clear the way for Horus's victory.
The plan nearly succeeded. The Ultramarines lost many thousands of warriors, and Calth was devastated. But the Ultramarines survived, and they fought back. The Word Bearers' treachery became a source of eternal hatred between the two Legions, and it is a hatred that endures to this day.
Heroes of Calth
The Battle of Calth produced many heroes. Among the most famous is Captain Remus Ventanus of the Ultramarines' 4th Company, who rallied the scattered defenders on the planet's surface in the first chaotic hours of the betrayal. With his company shattered and his commanders out of contact, he raised his voice over the vox and gathered every warrior who could hear him. His stand gave the Ultramarines a rallying point, and his name became legend. To this day, Ultramarines speak of him with reverence, and his deeds at Calth are taught to every new recruit.
Another hero of Calth was Sergeant Aeonid Thiel, a warrior whose unorthodox thinking repeatedly earned him reprimands from his superiors. During the battle, his willingness to break with convention saved many lives. His story is a reminder that even the Ultramarines, devoted as they are to discipline and doctrine, value initiative and courage. Thiel's legend grew after the battle, and he became a symbol of the Ultramarines' adaptability.
Many other heroes fell at Calth, their names recorded on memorials across Ultramar. Ordinary soldiers, Navy crews and civilians fought and died alongside the Ultramarines, and their sacrifice was not forgotten. Calth's memorials list countless names, and they are visited by pilgrims from across the realm.
Guilliman at Calth
Roboute Guilliman was at Calth when the Word Bearers struck. He had gathered his Legion there for the campaign against the Orks, and he had welcomed his brother Lorgar's sons as allies. When the betrayal came, he was caught as unprepared as his warriors. Yet he reacted with characteristic speed and clarity. Within hours, he had begun to organise the defence, and his leadership turned a rout into a fight.
Guilliman fought at the front in the battle's most desperate moments, and his presence inspired his warriors to fight on against impossible odds. When the battle was over, he was determined to learn from it. The experience of Calth, and of the wider Heresy, shaped his thinking as he later composed the Codex Astartes. It taught him that no Legion should ever be so large and concentrated that a single betrayal could destroy it.
The shipyards of Calth
Calth's orbital shipyards were among the greatest in the Imperium. They built warships for the Ultramarines and the Imperial Navy, and they were the pride of the realm. During the Battle of Calth, they were a primary target for the Word Bearers, and many were destroyed. Ships under construction were destroyed in their docks, and thousands of workers were killed.
After the battle, the shipyards were slowly rebuilt. They never fully regained their former glory, but they remain important to Ultramar. Today, they continue to build and repair warships, and they are heavily defended against any future attack.
Calth in the Era Indomitus
Ten thousand years after the Heresy, Calth remains a vital part of Ultramar. Its arcologies are home to millions, and its shipyards still produce warships for the Imperium. When Roboute Guilliman returned from his long slumber and the Great Rift tore the galaxy apart, Ultramar came under attack once more. During the Plague Wars, the Death Guard spread disease and corruption across the realm, and Calth's people prepared once again for war.
The legacy of the Battle of Calth is never far from the minds of the Ultramarines. The hatred between them and the Word Bearers is as strong as ever, and the two have clashed countless times across the millennia. For the Ultramarines, Calth is a reminder that betrayal can come from anywhere, even from those who call themselves brothers. For the people of Calth, it is a reminder that they have survived the worst the galaxy can throw at them, and that they can do so again.
People & culture
The people of Calth are a hardy and resilient folk. They have survived catastrophe and rebuilt their world in the darkness. They are proud of their history, and they honour the Ultramarines who saved them. They are also deeply suspicious of outsiders, especially those who claim to be allies.
Calth's culture is shaped by the memory of the battle. The people remember the betrayal and the suffering, and they are determined that it should never happen again. They are vigilant and disciplined, and they train for war from a young age. Many of them serve in the Ultramarines' auxiliary forces, or in the planetary defence forces.
The people of Calth also have a strong sense of community. Life in the arcologies requires cooperation, and the people work together to survive. They are proud of their arcologies, and they regard them as their homes, even though they are far from the open skies their ancestors knew.
Defenders & war-machine
Calth is defended by the Ultramarines and by its own forces. The Ultramarines maintain a strong presence on the planet, with fortresses and garrisons. The planetary defence forces are well trained and equipped, and they are supported by the Imperial Navy. The arcologies are heavily fortified, and the surface is guarded by defences.
Calth's shipyards are still important. They build warships for the Ultramarines and the Imperial Navy, and they are protected by orbital defences. Calth is one of the most heavily defended worlds of Ultramar, and any attacker would face a formidable challenge.
Notable battles
- The Battle of Calth (M31): the Word Bearers betray the Ultramarines; see Battle of Calth.
- The Underworld War (M31): the Ultramarines hunt the Word Bearers through the caverns of Calth.
- The Plague Wars (M42): the Death Guard assault Ultramar; see Plague Wars.
Trivia
- Calth's star, Veridian, was damaged during the Battle of Calth.
- The people of Calth have lived in underground arcologies ever since the Heresy.
- The Underworld War lasted for years after the main battle ended, as the Ultramarines hunted the last Word Bearers through the caves.
- The Battle of Calth is one of the most famous battles of the Horus Heresy.
- The Ultramarines had gathered at Calth to prepare for a campaign against the Orks of the Ghoul Stars.
- Remus Ventanus and Aeonid Thiel are among the most celebrated heroes of the battle.
