Far from the war-torn Cryptus system, the shadow of the Leviathan falls on a world under the protection of the Ultramarines. As his warriors lead a desperate rearguard to allow the world’s citizens to evacuate, Chief Librarian Tigurius tackles a monstrous new presence – a beast whose psychic powers threaten to overwhelm even him. Can he survive his first encounter with this creature, or will he be lost to the might of the hive mind? The Ultramarines are no strangers to fighting the tyranids, having suffered brutally at the claws of the galaxy's first hive fleet invasion. In this story, Chief Librarian Tigurius witnesses first hand the true scale of the malign intelligence that lurks at the core of the Hive Mind.