Dark Imperium by Guy Haley

Book 1 of the Dark Imperium trilogyThe 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 – with his armies of Primaris Space Marines – can hope to thwart the Death Guard's schemes.READ IT BECAUSEExperience the beginning of the Plague Wars, as the Imperium's lord regent fights to reclaim his home from the Death Guard and their daemonic allies.