Garro by James Swallow

From out of the shadows of the Silent War, a hero emerges. Clad all in grey, an errant warrior of the Legiones Astartes kneels before the Regent of Terra, and accepts a solemn new duty – Battle-Captain Garro, once commander of the Eisenstein, now Agentia Primus of Malcador the Sigillite. From the desolation of Isstvan to the halls of the Imperial Palace itself, he stands as a paragon of loyalty and protector of the innocent, ever ready to strike back at the traitorous allies of the Warmaster. But Garro is walking a path of his own, one that may lead him to question his own place in the Imperium... and what if he, too, should falter?