Event · M31
Slaughter at Calth
The Slaughter at Calth was the Word Bearers’ surprise assault on the Ultramarines at Calth, an early engagement of the Horus Heresy in the opening of M31. Calth, a world of the Veridian system within the Ultramarines’ realm of Ultramar, was the site of a muster of the XIII Legion’s forces. The Word Bearers were present as apparent allies of the Ultramarines. In fact they had already committed to the Warmaster Horus’s rebellion, and their Primarch Lorgar had turned to the worship of the Ruinous Powers. The attack was intended to cripple the Ultramarines and their Primarch Roboute Guilliman before the Legion could act against the rebels, and it is remembered as one of the earliest large-scale acts of open betrayal within the Space Marine Legions. The Word Bearers struck without warning while the Ultramarines were dispersed and unprepared. Their commanders included Kor Phaeron, the former foster-father of Lorgar, and the Chaplain Erebus, who is recorded as a principal agent of the conspiracy. The Captain Argel Tal also commanded on the ground. Accounts describe the Word Bearers using a Warp-touched device to alter Calth’s sun, which left much of the surface in darkness. Ultramarines officers such as Captain Remus Ventanus organised resistance, and the fighting continued in the cavern systems and settlements beneath the surface. The Ultramarines lost a large proportion of the forces on Calth, but they did not break, and the Word Bearers failed to destroy the Legion as intended. The engagement took place at about the same time as the Isstvan III atrocity and the Drop Site Massacre at Isstvan V, which made it one of several coordinated opening strokes of the Heresy. Guilliman was absent from Calth, and the Ultramarines’ survivors fought on as the Legion’s wider forces gathered in Ultramar. Calth was left scarred, and the Ultramarines’ losses shaped the Legion’s later part in the war, including the defence of Macragge and the region around Ultramar. The Word Bearers withdrew, and their survivors continued to serve Lorgar’s cause in the wider conflict. On the Orrery map, Calth marks the point where the war in Ultramar began. On the timeline, the battle sits alongside the events at Isstvan as the first phase of the civil war.
See where Slaughter at Calth occurred on the galactic map during the Horus Heresy.