Book 3 of the Dark Imperium trilogy 

The destinies of primarch and Imperial Regent Roboute Guilliman and his traitorous brother Mortarion come together for the climactic battle of the Plague Wars.

READ IT BECAUSE
It's the throwdown you've been waiting for, as Guilliman and Mortarion work out ten thousand years of pent-up resentment in a clash for the ages.