The story 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 return to the fray, taking 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, planning his revenge against his treacherous brother primarchs. 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.

Read it because
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.

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.