Nemesis

Book thirteen in the New York Times bestselling series After the horrors of Istvaan V, Horus declares outright war against the Imperium. In the shadows of the Emperor's Palace, powerful figures convene. Their plan is to send a team of assassins to execute the arch-traitor Horus and end the war for the galaxy of mankind before it's even begun. But what they cannot know is that Horus and his dark allies have already embarked on an equally sinister plan of their own.