Far on the Eastern Fringe, the realm of Ultramar has no equal. Having endured the Word Bearers' assault on Calth and the subsequent Shadow Crusade against the Five Hundred Worlds, the primarch of the Ultramarines, Roboute Guilliman, has gathered all loyal forces on Macragge, where he intends to deliberate on the future of humanity. With ever-greater numbers of people arriving to escape the war engulfing the rest of the galaxy, the line between friend and foe grows blurred. And now, with Terra cut off from Ultramar by raging warp storms, will Guilliman be able to muster enough strength to stand against the traitor, Warmaster Horus?