Overview
Calth’s people live beneath a surface their own sun makes lethal. The great cavern settlements shelter an industrial society that a passing ship might miss if its crew judged the world by its barren exterior. Above ground, the damage from the Heresy remains part of the weather.
The Word Bearers came to a joint muster and turned it into an attack on their supposed allies. The surviving population went below, where the fighting continued after the surface was lost. Calth still contributes ships and labour to Ultramar. Its people have made a working world under the ruin, with no prospect of simply walking outside to leave that history behind.
History
The Word Bearers attacked a joint muster during the Heresy, wrecking orbital facilities and poisoning Calth’s sun. Survivors withdrew into the caverns, where the Underworld War continued. The end of that struggle did not give them a surface they could inhabit again.
Military significance
Calth’s docks served fleet assembly before the Heresy and remain part of Ultramar’s industrial strength. Later attacks brought Tyranids, including Old One Eye, and Honsou’s Iron Warriors. Uriel Ventris is among the world’s sons who went into Ultramarine service.
M41 and M42 status
Inhabited; surface irradiated
Calth is inhabited underground and continues to contribute shipbuilding capacity. Its irradiated exterior covers a living society, unlike a world stripped of its population by Tyranid consumption.
Inhabited; later local condition unconfirmed
The world remains part of Ultramar, with civilisation sheltered below ground. The available sources do not establish a new destruction, evacuation or restoration of the surface after the Rift.
Timeline
Word Bearer betrayal poisons the sun and drives the survivors underground.
Behemoth attacks; Old One Eye becomes associated with Calth.
Honsou's invasion reaches Calth before the war concludes on Talassar.
Related charted locations
Administrative record
Tithe grade: Not established
Population: Not established
Chart placement and sources
Provisional placement
The official 2026 Inner Ultramar diagram places the Veridian system northeast of Macragge; Calth belongs to that system.
The planet uses its system's galactic marker; this does not locate Calth's orbit.
Official 2026 system diagram. The retained Macragge anchor and an editorial scale fit the local diagram into the existing Ultramar region. Relative layout is reproduced; physical distance and exact galaxy coordinates are not measured.
- Lexicanum — Calth — Secondary reference
Book references indexed by this source: Codex: Space Marines (5th Edition), p. 15; Codex: Tyranids (5th Edition), p. 57; The Chapter's Due, chapter 8, pp. 107–110
- Warhammer Community — Ultramarines Black Book extract — Official source
- Lexicanum — Mark of Calth — Secondary reference
Book references indexed by this source: Know No Fear, Ruin//Storm, chapter 4
- Assault on Castrum Velx, printed page 2 — Official source
- Warhammer Community: Inner Ultramar diagram, 11 February 2026 — Official source
- Ultramar — Lexicanum — Secondary reference
- Ultramar’s most iconic planets, 11 February 2026 — Official source
The dossier is available offline. Reference links open external websites.
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Cartographic positions and several administrative details are intentionally approximate where published Warhammer sources conflict or provide no canonical coordinate system.