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. The Night Lords, loyal to the Emperor, were sent to retrieve gene-seed of Legions long lost. On Sturmgarten, once the home of the XI Storm Riders Legion, they came into conflict with the Raven Guard under Corvus Corax, grudging allies of the Warmaster. Corax was speared by the Night Haunter's talons, Mercy and Forgiveness, while the Night Haunter lost his arm to the Raven's Claw. But the conflict was arrested by the realization that they had both been set up. Upon the Night Lords' return to Nostramo, the Primarch Lorgar punished the Night Lords by destroying their homeworld, and thus severed their last bonds to anything resembling restraint.
Meanwhile, the outnumbered Ultramarines, rebelling against the Emperor, try to hold onto their realm of Ultramar, under assault by the Imperial Legion of the Emperor's Children, without their own Primarch Guilliman there to lead them. But the Emperor's Children are degrading more and more into the worship of Slaanesh and away from the Emperor's perfection, a descent that nevertheless grants them great power. Defectors from the Emperor's Children have formed an independent Chapter - the Celestial Lions - and have sworn to aid the Ultramarines against their former brothers, and Marius Gage, the Ultramarine commander, has sent out a request for help. Ultramar will fight to the end... but at what cost? An unlikely alliance will be formed, vendettas and other plots will simmer, and a Legion will fall.
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.