Revealed at Adepticon 2023, Warhammer: The Horus Heresy – The Siege of Cthonia is the first campaign book for Warhammer: The Horus Heresy.

The book contains over 100 pages of narrative content focused on Horus’ home planet.

The planet of Cthonia is consumed by a rolling war and under siege by the Imperial Fists. Vheren Ashurhaddon, Master of the True Sons of Cthonia, vows to retake his home before Horus claims Terra, with the aid of the Word Bearers and the Alpha Legion.

Standing between the First Reaver and victory is a garrison of Imperial Fists led by Lord-castellan Evander Garrius. The Butcher of Larissan is reinforced by warbands of the Shattered Legions and even a cadre of Loyalist Thousand Sons.

There are rules for customizing forces for this campaign and features two sets of missions, five new Core Missions and two narrative-style Apex Missions.

There are also rules for Zone Mortalis with five missions and dedicated campaign rules, extra Imperial Fists and Sons of Horus rules and upgrades that can be used by any Legion.