Mark of Calth: The Horus Heresy, Book 25 (Unabridged‪)‬

The Heresy came to Calth without warning. In just a few hours of betrayal and bloodshed, the proud warriors of the XIIIth Legion - Guilliman's own Ultramarines - were laid low by the treachery of their erstwhile brothers of the XVIIth.

Now, as the planet is scoured by solar flares from the wounded Veridian star, the survivors must take the fight to the remaining Word Bearers and their foul allies or face damnation in the gloomy arcology shelters beneath the planet's surface.

In the novella-length Calth That Was, Graham McNeill highlights the fateful encounter that will alter the destiny of Captain Ventanus and shape the face of the Ultramarines Chapter for 10,000 years to come. The battle for Calth is far from over.