As the Indomitus Crusade begins, fleets of mighty warships leave Terra on a vital quest to stabilise Imperium Sanctus in the wake of the Great Rift. The returned primarch, Roboute Guilliman, leads a huge force towards the shrine world of Gathalamor, where stable warp routes will allow the flotilla to spread across the beleaguered southern half of the Imperium.But grave tidings reach the Imperial Regent's ears. Warnings from an ancient race, and eerie silence from the army tasked with holding Gathalamor until his arrival, lead Guilliman to send a reconnaissance mission to the world; at its head, Shield-Captain Achallor of the Adeptus Custodes.Achallor discovers a world on the brink: a beaten Imperial force and sinister agents of Abaddon the Despoiler who have unearthed an ancient evil; a weapon that when harnessed not only threatens the primarch, but perhaps the holy Throne of Terra itself...