Cthonia (meaning underworld or underground in Greek) was the homeworld of the Luna Wolves/Sons of Horus. Inhabitants of Cthonia had slightly freckled faces and pale, craggy features.
A mining world reachable from Terra at sub-light speeds (WHAT?), it was mined out by the Age of Strife and stripped of the mining equipment by the Mechanicum of Mars. There was literally nothing left to do but organize into loose gangs and stab each other. Horus landed here, but didn't stay on the planet long enough to conquer or fix it. As a youth, he discovered what would be the Talon of Horus on the advice of an old crone, pretty much the only other interesting thing that happened on this world and thus naturally not explored in the fluff.
The first Imperial contact with Cthonia took place in the early Great Crusade, when a Star Hunters pioneer company under Captain Kornelius Dure discovered the world and declared it a "nest of serpents coiling in the dark that we would be better to destroy". Which was true, actually, but not for the reason he thought. However, the planet was revealed as the homeworld of Horus, one of the Emperor's lost Primarch's and the first to be rediscovered. Upon incorporation into the Imperium, the Space Marines of the Luna Wolves Legion were created using the human inhabitants of the violent gangs inhabiting the planet's Hives. The Emperor found him while looking for recruits of good gene-stock, and the planet was left in anarchy to help train future Legion aspirants.
After the Horus Heresy, it got Exterminatus-ed in case something there might have contributed to Horus's corruption.
|The Planets, Systems, Regions and Sectors of the Galaxy|
|Imperial Homeworlds:||Holy Terra - Sacred Mars|
|Primarch Homeworlds:||Baal - Barbarus - Caliban - Chemos - Chogoris - Colchis |
Cthonia - Deliverance - Fenris - Inwit - Medusa - Nostramo
Nocturne - Nuceria - Olympia - Prospero - Macragge
|Notable Imperial Worlds:||Armageddon - Bakka - Cadia - Catachan - Cretacia - Ganymede |
Hydraphur - Necromunda - Phyrr - Pythos - Krieg - Kronus - Sacris
Sanctuary 101 - Scelus - Scintilla - Tanith - Tartarus - Titan - Vigilus
Vraks - Zayth - 108/Beta-Kalapus-9.2
|Daemon Worlds:||Plague Planet - Sortiarius - Sicarus - Medrengard - Glass Moon |
Fleshworld - Bathamor - Black Marble
|Systems and Regions:||Ghoul Stars - Halo Zone - Jericho Reach - Kaurava System |
Solar System - Stygius Sector - T'au Septs - Taelus System - Ultramar
|Types of Worlds:||Agri-World - Craftworld - Daemon World - Death World - Eldar World |
Forge World - Fortress World - Hive World - Civilised World - Tomb World