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Calth is part of Ultramar, the Realm of the Ultramarines. It is an airless world, and its surface is uninhabitable due to the deadly light of its blue sun. Despite this barren appearance, it is one of the most productive worlds in Ultramar; its population inhabits massive underground cavern-cities, and its orbital shipyards are renowned throughout the Imperium.
By the end of the Great Crusade, Calth had become one of the most important worlds of Ultramar alongside Macragge, Saramanth, Konor, Occluda, Iax and Armatura.[6a] It was believed that Calth would soon control its own fiefdom of Ultramar, with its own Tetrarch ruling the planet.[6c] There were even plans to install a superorbital plate above the planet.[6a]
The Horus Heresy
Some time after the primarch Horus was named Warmaster of the Great Crusade by the Emperor of Mankind, he ordered Roboute Guilliman and Lorgar Aurelian to prepare their Legions (the Ultramarines and Word Bearers, respectively) for a planned campaign against the orks of the Ghaslakh xenohold. Calth was chosen as the mustering world for the crusade, due to its extensive orbital docking network.[6a]
However, this was a ruse on Horus' part, done in preparation for his great rebellion against the Imperium. In one of the first actions of the Horus Heresy, the Word Bearers (who, having fallen to Chaos, had joined Horus' Traitor Legions), launched an ambush against the Loyalist Imperial forces on Calth, hoping to cripple the Ultramarines so they would not be able to stop Horus.[6x] Eventually the Ultramarines and other Loyalist forces emerged victorious, but millions of civilians died in the Word Bearers' cataclysmic attack, which permanently stripped away Calth's atmosphere and poisoned the Veridian sun's radiation, making it impossible for any life to survive above ground without shelter. Thereafter, Calth's entire population occupied the vast cavern cities, with the exception of a few heavily shielded starports and surface cities.[5b]
War of the Beast
Hive Fleet Behemoth
Hive Fleet Behemoth's invasion of Calth was spearheaded by Old One Eye, a unique Carnifex with the ability to heal from mortal wounds. Its rampage was finally stopped when a forgotten hero of the Imperium fired a Plasma Pistol through its eye and into its brain. Decades after Behemoth had been driven from Calth, a band of smugglers stumbled across Old One Eye's body frozen intact in the ice-packs of Calth. They thawed it out hoping to get a bounty for the corpse, but Old One Eye's grievous wounds instantly began to heal. It soon awoke and slaughtered the smugglers. A series of Tyranid raids followed. Land conveys were destroyed, hab-domes smashed and entire populations massacred. Lesser Tyranid creatures such as Termagants and Genestealers, also left behind from Behemoth's invasion, were drawn to Old One Eye's presence as a dominant leader. The people of Calth cried out for aid, and they were answered by Scout Sergeant Telion of the Ultramarines. Telion tracked down Old One Eye, but found it difficult to pierce its armoured flanks. As Telion's warriors were cut to pieces by the beast, Telion made a one-in-a-million shot that hit its ruined eye-socket. Old One Eye was overcome with pain and stumbled into a cavernous ravine. Telion led a search for the body, but Old One Eye was never found. There have since been several reports of Carnifexes matching its description causing havoc across Ultramar and beyond.
Invasion of Ultramar
Calth was the location of some of the fiercest fighting of the Invasion of Ultramar by the Chaos warband known as the Bloodborn. The planet was attacked by a Grand Company of the Iron Warriors and a host of other mercenaries, led in person by Warsmith Honsou. Honsou had been "ordered" by the Daemon Prince M'kar to find and destroy the tomb of Remus Ventanus, who had led the defense of Calth during the Heresy. Entombed with Ventanus was the Shard of Erebus, a strange ancient weapon that had the power to destroy M'kar. Honsou's attack failed, but Captain Ventris retrieved the weapon and took it to Talassar, where M'kar was destroyed permanently.[5x]
- Boros River[6a]
- Caren Province[6b]
- Dera Caren Lowlands — Devastated during the Battle of Calth.
- Erud Province[6b]
- Guilliman's Gate.[5a]
- Lanshear — City-sized starport. Devastated during the Battle of Calth.
- Leptius Numinus.[6d]
- Mercius — Devastated during the Battle of Calth.
- Numinus City — Devastated during the Battle of Calth.
- Ourosene Hills[6b]
- Persephone Gate.
- Ultimus Prime — Also known as Highside City.[5b]
- Vollard Meadows — Agri-region.
Notable Scions of Calth
- Uriel Ventris, Space Marine Captain of the Ultramarines 4th Company.
- Pasanius Lysane, Space Marine Sergeant of the Ultramarines 4th Company.
- 1: Codex: Space Marines (5th Edition), pg. 15
- 2: Codex: Tyranids (5th Edition), pg. 57
- 3: Nightbringer (Novel), [Needs Citation]
- 4: Defenders of Ultramar (Graphic Novel), Chapter One
- 5: The Chapter's Due (Novel)
- 6: Know No Fear (Novel)
- 7: Honour to the Dead (Audio Drama), Track 1
- 8: Echoes of the Long War (Novel), Chapter 3
- 9: Codex: Space Marines (5th Edition), pgs. 12–15
- 10: Betrayer (Novel)[Needs Citation]
- 11: Codex: Space Marines (8th Edition), pg. 25
- 12: Calth That Was (Novella) — Mark of Calth (Anthology), Section II
- 13: Censure (Short Story) — Legacies of Betrayal (Anthology)