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. Horus Lupercal's own Luna Wolves lead a coalition of eight Legions that have declared open rebellion against the Emperor; two others have privately sided with the rebellion. On Terra's side, eight more Legions have remained loyal to the Imperium.

In the wake of the destruction of the Thousand Sons' homeworld of Prospero, a galactic war has begun in earnest. The Emperor seeks to ascertain the loyalties of the White Scars and the Alpha Legion, who have not made their allegiance clear. Vulkan, the Primarch of the Salamanders – once the most compassionate of the eighteen – is becoming more and more twisted by the whispers of a daemonic hammer. And in the cradle of humanity, under the Emperor's own feet, three legends begin a journey of their own.

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.