Fear to Tread by James Swallow

Since the earliest days of the Great Crusade, Sanguinius - angelic primarch of the IXth Legion - was ever among the closest and most loyal of Horus's brothers. But the Blood Angels have long kept their true nature hidden from the rest of the Imperium, and when the Warmaster hints that the key to their salvation may lie in the ruins of a conquered world, the sons of Sanguinius race to claim it. Now, as the revelation of their betrayal dawns and the traitors' hand is revealed, the Blood Angels must face all the warp-spawned armies of Chaos, as well their own personal daemons, upon the blasted plains of Signus Prime...