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. The hordes of his traitorous brother, Mortarion, march on Ultramar, and only Guilliman can hope to thwart their schemes with his Primaris Space Marine armies.

READ IT BECAUSE
Experience 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.