A strike force of Grey Knights led by Castellan Garran Crowe is sorely tested when the planet they are fighting on is swallowed by a mighty warp storm.

For more than a century, Garran Crowe has served as Castellan of the Grey Knights, protecting the Imperium against the foulest daemons, and holding prisoner in his grasp the monstrous Black Blade of Antwyr. Now he is tested as never before when the planet he and his strike force are fighting on, Sandava III, is swallowed by the mighty warp storm known as the Cicatrix Maledictum. But their struggle for survival is only just beginning, for when the Grey Knights return to a transformed Imperium, they discover that the fate of Sandava III is just one part of a more monstrous danger. Crowe must fight an enemy whose patiently laid plans have at last come to fruition, threatening the Imperium with inconceivable destruction.