Event · M31
Siege of Cthonia
The Siege of Cthonia was a Horus Heresy campaign of 013–014.M31 fought for Cthonia, Horus’s homeworld and the recruiting ground of the Luna Wolves and, later. It fell in the last phase of the Heresy, after the Defence of Ryza and immediately before the Siege of Terra itself, as loyalist forces moved against the Warmaster’s own ground in the approaches to the Sol system. The Imperial Fists under Rogal Dorn maintained a blockade of Cthonia on its Terra-facing side, denying the mined-out gang world an open path to the Imperium’s capital. Ground and Titan forces engaged on both sides: elements of the Traitor-aligned Legio Magna fought to hold the planet, while House Vyronii’s Knights supplied the majority of the loyalist Knight strength committed to the siege. Cthonia’s own population, organised for generations into tunnel gangs, split with the war; accounts hold that some of these gangs took up the loyalist cause before the fighting ended, against others who remained loyal to Horus. The siege closed with the arrival of the Dark Angels under Lion El’Jonson, who destroyed the planet in 014.M31 rather than leave it as a base for the Sons of Horus. That destruction, recorded separately as the Destruction of Cthonia, ended the world’s long role as a source of recruits for Horus’s Legion and struck it from the Imperium’s territory in the same year the Siege of Terra was fought. The campaign is remembered chiefly for what it took from Horus rather than for ground held: a homeworld, a recruiting ground, and a symbol, denied to the Warmaster as his own siege of Terra began.
See where Siege of Cthonia occurred on the galactic map during the Horus Heresy.