Perturabo is the siege master and executioner of Olympia. For long he lived in the shadow of his more privileged brother Primarchs, disillusioned by the mundane and shameful duties that regularly befell his Legion. When Fulgrim offers him the opportunity to lead an expedition in search of an ancient and destructive xenos weapon, the Iron Warriors and the Emperor's Children unite in a venture deep into the heart of the maelstrom of stars, following Perturabo's dreams. Haunted by revenge-seeking survivors of Isstvan V and the ghosts of the dead Eldar world, they must act quickly if they are to unleash the destructive power of the Angel Exterminatus!