It is a period of failing light. Warmaster Horus Lupercal observes dark changes in the Imperium of Man he serves.

For the nigh-immortal Emperor of Mankind has struck a grim bargain with the four Chaos Gods, eldritch nightmares thirsting for human suffering. The Space Marines, once the Imperium's finest soldiers, are turning into monsters. Most monstrous of all are the bloodthirsty World Eaters, and they are now heading to the Thousand Sons' home world of Prospero, to destroy it utterly.

But not all of the Space Marines, and their leaders, the ultrahuman Primarchs, remain loyal to the Imperium. Horus Lupercal and nine others were not privy to the Emperor's plans. As the shock settles over them, and plans of rebellion are drafted, two Primarchs head to battle. Magnus the Red of the Thousand Sons has seen the doom of his home, and together with Leman Russ of the Space Wolves, he hurries back. The savage violence of the Space Wolves, the gladiatorial madness of the World Eaters, and the mystical power of the Thousand Sons converge, and the stage for a battle of brothers is set.

A new path for the galaxy is open, paved with the ashes of worlds. The age of debate and enlightenment is over, but the dream of empire remains.

Only now, it is a black dream.