In this all-new collection of Horus Heresy stories, witness the untold tales of the Underworld War. The Heresy reached 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 shelters beneath the planet’s surface. The battle for Calth is far from over...