Galaxy in Flames (Horus Heresy Series #3) by Ben Counter

Having made a miraculous recovery from the grevious injuries he suffered on Davin, Warmaster Horus now leads the triumphant Imperial forces against the rebel world of Isstvan III. An unprecedented alliance between the Sons of Horus, Death Guard, World Eaters and Emperor' Children Legions seems more than capable of overwhelming the paltry mortal defences - indeed, such a display of force seems unneccesary? Putting their own concerns aside, Garviel Loken and his loyal kinsmen lead their companies to the surface only to learn the full, horrifying truth, and the legendary war known as the Horus Heresy begins with the most foul act of betrayal imaginable...