Istvaan III, also spelled as Isstvan III in Imperial records, was an Imperial Civilised World in the Istvaan System of the Segmentum Obscurus. Its destruction by a virus-bombing ordered by the Warmaster Horus and the internecine battle that followed between the Sons of Horus, Emperor's Children, World Eaters and Death Guard Traitor Legions and their own Loyalist elements within the ruins of the planetary capital Khry Vanak (translated as the "Choral City" into Imperial Low Gothic) was the first overt sign of Horus' turn to Chaos, and the first battle of the Horus Heresy in the early 31st Millennium.