Cthonic System

The Cthonic System is a mined-out star system in Segmentum Solar, close to Terra, in which Horus was found and the world of Cthonia once lay. A gang-world, Cthonia supplied recruits from its tunnels to the Luna Wolves and later to the Sons of Horus, the XVI Legion of the Space Marines.

The system was never a civilised capital but a resource-extraction site, whose gangs produced capable legionaries. Its proximity to Sol made it politically sensitive once those legionaries turned against the Emperor in the Horus Heresy. After the Heresy, the Dark Angels destroyed Cthonia, leaving the system depleted and empty.

Later maps retain the system because the origin of the XVI Legion lies in a mining world close to Terra rather than on a distant, primitive one. It therefore remains a fixed point on charts of the Heresy, although the planet no longer exists. By convention, the system is spelt Cthonic and the world Cthonia, and the system is placed within Segmentum Solar.