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. But questions remain, and the new course of the Imperium is clear to none. Loyalties arer in flux, shaken by each witnessed atrocity.

Eight of the Space Marines' eighteen Primarchs have renewed their oaths to the Emperor, and to evil. But another, Magnus the Red of the Thousand Sons, has seen the Emperor's new way with his psychic powers; and so he contacts the Warmaster, Horus of the Luna Wolves, greatest of the Primarchs, who is furious that the Emperor is subverting his authority over the Great Crusade. Three companies of Space Marines are sent to the planet of Venus IX, to test the veracity of Magnus's claims. What they find does not bode well.

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.