Censured at the Council of Nikea for his flagrant use of sorcery, Magnus the Red and his Thousand Sons Legion retreat to their homeworld of Prospero to continue their use of the arcane arts in secret. But when the ill-fated primarch foresees the treachery of Warmaster Horus and warns the Emperor with the very powers he was forbidden to use, the Master of Mankind dispatches fellow primarch Leman Russ to attack Prospero itself. But Magnus has seen more than the betrayal of Horus and the witnessed revelations will change the fate of his fallen Legion, and its primarch, forever.

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One of the most tragic tales of the entire Horus Heresy unfolds, as the Thousand Sons are brought low by their relentless quest for knowledge. Magnus is sure that he knows better than the Emperor, but isn't that what caused Horus to fall as well?

The eBook edition includes integrated illustrations of characters and events from the story, and an afterword that places the story in context within the series.