Cthonia

Cthonia was a mined-out gang world near Terra, known as the place where Horus was found and as a recruiting ground for his Legion. Its population lived in the tunnels of the exhausted mining world and organised into gangs.

The Luna Wolves, later renamed the Sons of Horus, drew recruits from these tunnels. During the Horus Heresy, the Imperial Fists under Rogal Dorn maintained a blockade of the planet on its Terra-facing side, and some of its gangs fought for the loyalist cause before the end. The Dark Angels under Lion El’Jonson destroyed the planet in 014.M31, that is, in year 014 of the thirty-first millennium.

Cthonia’s standing rests on three points: its proximity to Terra, its connection with the discovery of Horus, and its role as a source of recruits for the Luna Wolves and the Sons of Horus. Its destruction by the Dark Angels ended that role and removed the planet from the Imperium’s territory.