Overview
The mines on Cthonia had been worked almost empty before Horus’s Legion began taking men from them. Gangs fought in the tunnels left behind, beneath a surface fouled by industry. A recruit could leave the warrens and return as one of the most powerful soldiers in the galaxy. The Legion found plenty who were willing.
Cthonia lay close to Terra, an exhausted world with a claim on the Warmaster that its remaining minerals could never have earned. The Luna Wolves drew on its people; the Sons of Horus carried its name into rebellion. The later destruction of the planet ended its usefulness as a recruiting ground. It did considerably less to end the loyalties learned there.
History
The Great Crusade took recruits from Cthonia’s gangs into Horus’s Legion. During the Heresy, Imperial Fists established positions on the homeworld and fought Sons of Horus forces returning to recover it.
Some inhabitants supported the loyalists. The divisions on the ground were more complicated than the later account of an entire traitor world might suggest. Cthonia was destroyed at the end of the Heresy.
Military significance
Vheren Ashurhaddon’s Sons of Horus fought Evander Garrius’s Imperial Fists in the Siege of Cthonia. Word Bearers and Alpha Legion forces also took part, alongside Shattered Legion warriors and loyalist Thousand Sons. The planet drew combatants whose allegiances could not be identified safely from Legion colours alone.
M41 and M42 status
Destroyed; historical location
Cthonia is destroyed. Its mines and recruitment grounds belong to the Legion’s past. There is no continuing industrial society established at the former homeworld.
Destroyed; no restoration established
No restoration is recorded. Cthonian identity persists among descendants and inherited traditions, without a recovered planet to receive them.
Timeline
Cthonia supplies recruits to Horus's Legion.
Imperial Fists and Sons of Horus fight successive sieges.
Dark Angels destroy Cthonia.
Related charted locations
Administrative record
Tithe grade: Not applicable
Population: Original planetary population lost
Chart placement and sources
Provisional placement
The official 2025 Carta Galactica places Cthonia southeast of Terra in the Solar region.
The retained point expresses proximity to Terra; map spacing is not a distance in light years.
Source-reviewed regional estimate in the existing 0–1000 chart. The retained galaxy anchors, exact point and any ring size are not surveyed coordinates or territorial boundaries.
- Lexicanum — Cthonia — Secondary reference
Book references indexed by this source: The Horus Heresy Book One: Betrayal, p. 65; White Dwarf 470, p. 105; Campaigns of the Age of Darkness: The Siege of Cthonia, pp. 42–43 and 56–57
- Warhammer Community — Decide the Fate of Cthonia — Official source
Book references indexed by this source: Campaigns of the Age of Darkness: The Siege of Cthonia
- Carta Galactica: Imperium of Man, official 2025 product preview — Official source
- Cthonia — Lexicanum — Secondary reference
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.