The Treachery at Port Maw was a notable military defeat for the Imperium of Mankind during the earliest years of the Horus Heresy and what was later designated the Manachean War. It was by the twin assaults of the Traitors on Hive Illum and the Armada Imperialis Port Majoris of Port Maw that the Manachean War was concluded, marking the end of the powerful star system that had been the heart of the Manachean Commonwealth and the establishment of the first Traitor-held domain in the galaxy. As far as the dire events of Istvaan III and Istvaan V revealed Horus' influence within the Legions of the Legiones Astartes, the Treachery at Port Maw is a testimony to how far Horus' treason ran within every other organisation -- military and civilian -- of the Imperium. To Imperial historians, the "Tre