Cthonia (meaning underworld or underground in Greek) was the homeworld of the Luna Wolves/Sons of Horus. A mining world reachable from Terra at sub-light speeds, 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 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 - Ultramar
|Planets:|| Armageddon - Cadia - Catachan - Ganymede |
Krieg - Kronus - Sacris - Sanctuary 101 - Scelus
Tanith - Tartarus - Vigilus - Vraks - Zayth
|Daemon Worlds:||Plague Planet - Sortiarius|
|Systems and Regions:||Solar System - Jericho Reach - Kaurava System |
Halo Zone - Stygius Sector - Taelus System
|Types of Worlds:|| Agri-World - Craftworld - Daemon World - Death World - Eldar World |
Forge World - Fortress World - Hive World - Tomb World