It is a period of civil war. Rebel spaceships, led by the former imperial Warmaster Horus, are beginning their campaigns against the corrupted Imperium of Man.
Against them, the nigh-immortal Emperor waits on his Golden Throne. Allied with him are the four Chaos Gods, eldritch nightmares thirsting for human suffering. The Space Marines, once the Imperium's finest soldiers, are divided. With the breaking of the bond between Horus and the Emperor, even the strength of Legion brotherhood splinters under the pressure. For that brotherhood was held together by simple loyalty, and in these days, with the universe full of evil and doubt, no loyalty is simple.
Among Horus's Primarch brothers who have aligned with his rebellion are Mortarion of the Death Guard and Perturabo of the Iron Warriors, neither possessed of the most savory reputation. Now, Horus dispatches them to lead the armies that hope to save humanity from the laughter of thirsting gods. The coldly logical Perturabo prepares for war by studying every detail of the changing tactics of his greatest rival - Rogal Dorn, the Emperor's Champion - and his Imperial Fists Legion, for he knows that the two of them, siegemasters both, will face each other in due time. Mortarion, wrapped in a cloak of ill omens, remembers the pict-film he discovered of the World Eaters and Salamanders purging their own ranks of those doubting the Emperor, and the dead bodies memorializing that struggle. Yet he cannot perceive the echo that massacre is creating.
The screams and pleas of the innocent will have no effect - not anymore. The age of debate and enlightenment is over. The dream of empire has ended.
The nightmare has begun.