The Istvaan III Atrocity, also known as the Battle of Istvaan III to Imperial scholars, was the treacherous military campaign conducted by the Chaos-corrupted Warmaster Horus on the rebellious Civilised World of Istvaan III that served both to amass those forces loyal to him without suspicion, and also cull those elements within the ranks of the Traitor Legions whose loyalties he suspected as remaining true to the Emperor of Mankind. The destruction of Istvaan III by virus-bombing and the subsequent internecine battle that followed between the Sons of Horus, Death Guard, Emperor's Children and World Eaters Traitor Legions and their own Loyalist elements was the first overt sign of Horus' turn to Chaos, and the first battle of the Horus Heresy in the early 31st Millennium.