Istvaan V, also spelled as Isstvan V in Imperial records, was the barren Imperial world in the Istvaan (Isstvan) System of the Segmentum Obscurus where the infamous Drop Site Massacre of the Horus Heresy took place that deeply damaged the Loyalist Space Marine Legions in the worst betrayal in Imperial history. During the massacre, the Traitor Legions of the Warmaster Horus redeployed following the virus-bombing of the Traitor Legions' own Loyalist members at Istvaan III at the start of the Horus Heresy. Upon learning of the terrible atrocity Horus had committed, the Emperor of Mankind ordered seven Loyalist Legions of Space Marines deployed to the Istvaan System to bring Horus to account for his actions. The Iron Hands, Salamanders and Raven Guard made up the first wave of the attack, but