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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.
The world of Calth had maintained contact with its neighboring systems, including Macragge, allowing it to largely survived the chaos of the Age of Strife. Early in the rise of Roboute Guilliman, Calth became part of the new Ultramar empire. When Guilliman met the Emperor, Calth became part of the greater Imperium. By the end of the Great Crusade, Calth had become one of the three most important worlds of Ultramar alongside Macragge and Armatura.
The Horus Heresy
During the Horus Heresy, Calth was used by Roboute Guilliman as a staging point for his supposed assault on the Veridian System. Horus had ordered the Ultramarines there to divert them from the Siege of Terra. It was there that they were attacked by a force of the Word Bearers legion. 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]
- Dera Caren Lowlands — Devastated during the Battle of Calth.
- Guilliman's Gate.[5a]
- Lanshear — City-sized starport. Devastated during the Battle of Calth.
- Leptius Numinus.[6b]
- Mercius — Devastated during the Battle of Calth.
- Numinus City — Devastated during the Battle of Calth.
- Persephone Gate.
- Ultimus Prime — Also known as Highside City.[5b]
- Vollard Meadows — Agri-region.
- 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)