Deep in the Solar Segmentum, on the world they swore to protect, the honor guards of the Imperial Fists are dying. The mysterious cult responsible is gaining strength day by day, drawing into its sinister networks the inhabitants of the planet Gair - from the poor miner to the carefree aristocrats. The Imperial Fists send Primaris Librarian Aster Lidorran and his zealous brothers to combat the threat. But the siege masters are faced with an insidious enemy they have never faced before. As more and more dark omens emerge, Lidorran finds himself drawn into a battle of wits and wills with an adversary with psychic power that may be greater than that of the Librarian. And the enemy is patient - after all, every hour Gair is approaching death. The unshakable sons of Dorn will have to leave their native walls and accept the help of new allies in order to prevent a catastrophe on the very threshold of Terra.