Book 1 of the Dark Imperium trilogy

The first phase of the Indomitus Crusade is over, and the conquering primarch, Roboute Guilliman, sets his sights on home. His traitorous brother, Mortarion, marches on Ultramar, and only the primarch of the Ultramarines â?? with his armies of Primaris Space Marines â?? can hope to thwart the Death Guard's schemes.