Angel Exterminatus (The Horus Heresy) by Graham McNeill

Perturabo – master of siegecraft, and executioner of Olympia. Long has he lived in the shadow of his more favoured primarch brothers, frustrated by the mundane and ignominious duties which regularly fall to his Legion. When Fulgrim offers him the chance to lead an expedition in search of an ancient and destructive xenos weapon, the Iron Warriors and the Emperor’s Children unite and venture deep into the heart of the great warp-rift known only as ‘the Eye’.