The Soul, Severed: The Horus Heresy Series

As Lord Commander Primus, Eidolon leads the Emperor's Children in the absence of his primarch - who even knows where the daemon prince Fulgrim resides now? But a challenge from within the Legion forces Eidolon to confront one of his rivals, the ambitious Archorian, a capable officer and tactician who would see the Legion returned to glory. Dissent is unacceptable. It is time for the Kakophoni to be set against their erstwhile brethren.