Book 18: Deliverance Lost
A Horus Heresy novel
After their near-destruction at the hands of Horus’ Traitor Legions, the Raven Guard seek to escape and rebuild, but the forces of Chaos are insidious, and the Legion may yet face its doom.
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It's an account of a Legion battered and broken after the events of Isstvan V and the disastrous results of Corax's attempt to rebuild and rejoin the fight.