A Great War is coming, and it will engulf the Imperium of Man. The Space Marines of the Alpha Legion, the last and most secretive of all the Astartes brotherhoods, arrive on a heathen world to support the Imperial Army in a pacification campaign against strange and uncanny forces. But what drives the Alpha Legion? Can they be trusted, and what side will they choose when the Heresy begins? Loyalties are put to the test, the cunning schemes of an alien intelligence are revealed, and the fate of mankind hangs in the balance.