As the Horus Heresy divides the Imperium, Corax and his few remaining Raven Guard escape the massacre at Isstvan V. Tending to their wounds, the bloodied Space Marines endeavour to replenish their numbers and take the fight to the traitor Warmaster. Distraught at the crippling blow dealt to his Legion, Corax returns to Terra to seek the aid of his father - the Emperor of Mankind. Granted access to ancient secrets, Corax begins to rebuild the Raven Guard. But not all his remaining warriors are who they appear to be... the mysterious Alpha Legion have infiltrated the survivors and plan to destroy the Raven Guard before they can rebuild and threaten Horus's plans.