Calth's inhabitants live in subterranean hive cities where the deadly ultraviolet light of Calth's blazing blue sun cannot reach them due to the planet's lack of an atmosphere. The caverns of Calth are constructed on such a huge scale, and with such grandeur, that they are as light and airy as any city of Macragge.
The world above is an arid wasteland devoid of air and covered in the ruins of shattered cities. The atmosphere of the world was stripped away during a nuclear bombardment of the planet's hive cities during the terrible Battle of Calth that raged over 10,000 Terran years ago in the Horus Heresy. This was when the Ultramarines and the Word Bearers Traitor Legion, under the command of First Captain Kor Phaeron and the Dark Apostle, Erebus, came into conflict.
Although the inhabitants of Calth are perfectly capable of living on what is grown in their subterranean algal nutrient vats, they prefer to import fresh foodstuffs from the nearby Agri-World of Iax. The orbital shipyards of Calth also earn the world a good reputation as Calthian-made starships are used by the Ultramarines and by merchants, Rogue Traders and the armed forces of the Imperium at large. The Ultramarines' Captain Uriel Ventris was born upon Calth.
The Tyranid remembered as Old One Eye was discovered on Calth frozen in ice. At least one regiment of the Imperial Guard has been raised on Calth and the world is protected by the Ultramar Auxilia, the regiments of the well-trained Planetary Defence Forces of the Realm of Ultramar.
Calth was originally a verdant Agri-World whose manufacturing output at the time of the Great Crusade would have rivalled that of Macragge in only two to three more solar decades if the Horus Heresy had not intervened to push the planet down a radically different developmental path. 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 at the time, which were marvels of human technology and engineering and symbols of the new Golden Age that the Emperor intended to create for Mankind.
On the eve of the Calth Conjunction before the start of the Battle of Calth, Calth was held by many as an as-yet uncut jewel in the crown of Ultramar, a glittering example of the future. It was long planned that Calth would take its place amongst the greatest of the Five Hundred Worlds and that its name would be known not just within Ultramar, but across the entire Imperium of Mankind.
To that end, since its settlement just a few short generations before the outbreak of the galactic civil war, Calth's infrastructure was rapidly expanded in expectation of future growth. Many of its cities were built as mighty arcologies -- self-sustained centres of habitation and production. Many of these arcologies were built within the extensive network of subterranean chambers and tunnels for which the world had become renowned and many housed millions of workers. These centres of population were naturally protected from the extra-seasonal solar storms which assailed the world approximately every fifteen years, and hosted additional shelters to accommodate surface dwellers forced below during especially heavy periods of coronal massed ejection.
In orbit above Calth was one of the most extensive orbital docking, repair and resupply facilities in the entire Five Hundred Worlds and beyond. This facility was built in expectation of future expansion far beyond the extent of Ultramar, and was due to see its most auspicious use to date with the Calth Conjunction -- the mustering of the combine fleets of the Legiones Astartes Word Bearers and Ultramarines in preparation for a joint attack against the Orks of Ghaslaskh xeno-hold. Being a strategically vital node in the infrastructure of the Five Hundred Worlds, Calth was heavily protected by a planetary defence network far in excess of most Imperial planets. This grid integrated over nine hundred orbital defence platforms and thousands of ground cities, from defence laser silos to interceptor launch pads, into one of the most potent defensive networks in the entire Segmentum.
The true value of Calth to the Five Hundred Worlds, and the reason for the massive investment in resources committed to it; was as a potent symbol of Mankind's future. As the Great Crusade drew to a close, the Primarch Horus taking the mantle of Warmaster while the Emperor of Mankind returned to Terra to oversee the next stage in the expansion of the Imperium, great leaders such as Lord Guilliman looked to their own role in that future. The people of Ultramar were immeasurably proud of the realm they had built and looked to a bright future in which Ultramar and the greater Imperium watched benevolently over a galaxy re-shaped by the sacrifice and toil of two centuries of conquest. 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. It stands as one tragedy among countless others that this future would never be realised, due to the perfidy of Warmaster Horus and the Traitor Legiones Astartes.
Battle of Calth
It is unlikely that the true architect of the Calth atrocities will ever be known however, and weighed against the innumerable sins committed by both Lorgar and Horus, the attribution of this one offence is inconsequential. What is known is that the earliest stages of the assault on Ultramar were laid down long before Horus arrived at the fateful worlds of Istvvan, in the year 005.M31, with a series of orders issued under the seal of the Warmaster dispatching a number of Legions to campaigns in the furthest reaches of the Imperium. Of these, the Blood Angels were sent forth in their entirety to Signus, the Dark Angels to Tsagualsa and Ultramarines would muster alongside the Word Bearers at Calth. 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 XIIIth and the XVIIth 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 XIIIth 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 XIIIth Legion had destroyed one of the Word Bearers' greatest achievements, the sacred city of Monarchia, four decades earlier.
Command of the main assault force was given to Kor Phaeron, the First Captain of the XVIIth 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 XIIIth 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.
Though initially caught off guard by the treacherous surprise assault, the Ultramarines successfully broke the Word Bearers' attack 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 arcologies 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.
Hive Fleet Behemoth
Old One Eye
Hive Fleet Behemoth's invasion of Calth had been led by a unique and extremely powerful Carnifex bioform nicknamed "Old One Eye" by the Imperial defenders that had the power to continually regenerate its cells, even from seemingly mortal wounds. The Carnifex was defeated by an anonymous Imperial hero who shot it through the eye with a Plasma Pistol. Decades later, a band of smugglers discovered Old One Eye's body frozen in a glacier in Calth's arctic region. They thawed the body out hoping to earn a reward for discovering the famous Tyranid and selling it to the highest bidder, but Old One Eye's regenerative powers had remained active, if slowed by the cold, and it awakened and butchered its rescuers.
As soon as it returned to activity, scores of Tyranid creatures who still existed as feral beasts in the Calthian wilderness, including Genestealers and Termagants, were psychically drawn to Old One Eye's presence and the beast began to direct a series of Tyranid raids against Calthian convoys and hab-domes, killing thousands of people. As the people of Calth called out for deliverance to the Ultramarines, the Chapter sent Scout Sergeant Torias Telion and his squad of Scout Marines to handle the matter and prove themselves worthy of becoming full Initiates. Telion's squad tracked down Old One Eye, but the beast and its Tyranids began to slaughter Telion's squad. In a moment of desperation, Telion fired an aimed shot from his Bolter that struck the beast's already ruined eye-socket where it had been killed the first time.
The creature wandered off in pain and disappeared into a ravine where its body was never found. With Old One Eye's second death, its Tyranid swarm was quickly destroyed by the Ultramarines since they were now leaderless. But reports later emerged of a powerful Carnifex fitting Old One Eye's description leading assaults on other worlds across the Ultramar Sector and beyond, indicating that the creature not only survived but somehow managed to escape off of Calth.
- Codex: Space Marines (5th Edition), pg. 15
- Codex: Tyranids (5th Edition), pg. 57
- Codex: Ultramarines (2nd Edition), pg. 14
- Horus Heresy: Collected Visions, pp. 153, 158, 162-164
- The Horus Heresy - Book Five: Tempest by Alan Bligh, pp. 17-18, 20-21, 24-43, 50-69, 71, 79, 82, 102, 105, 109, 114, 119, 123, 125-127, 133-135, 139-143, 145, 147, 152-154, 156-158, 164-175, 177
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