Fulgrim by Graham McNeill

Under the command of the newly appointed Warmaster Horus, the Great Crusade continues. Fulgrim, Primarch of the Emperor's Children, leads his warriors into battle against a vile alien foe, unaware of the darker forces that have already set their sights upon the Imperium of Man. Loyalties are tested, and every murderous whim indulged as the Emperor's Children take their first steps down the road to true corruption - a road that will ultimately lead them to the killing fields of Isstvan V...