Istvaan V

Istvaan V was the world of the infamous Drop Site Massacre where 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 deployed seven Loyalist Legions of Space Marines to bring Horus to account for his actions. The Iron Hands, Salamanders and Raven Guard made up the first wave of the attack, but were pushed back by the superior tactics of Horus' maddened Chaos Space Marines. The Loyalist reserves were called in, but the four Space Marine Legions comprising the assault's reserves - the Iron Warriors, the Alpha Legion, the Word Bearers and the Night Lords -- had also secretly betrayed the Emperor and were prepared to follow Horus and swear themselves to Chaos. The three Loyalist Legions were almost annihilated in the resulting crossfire but several thousand survivors from each Legion managed to escape off-world, though they were too decimated to play much of a further role in the defence of the Imperium from Horus' betrayal.

The Drop Site Massacre
At the start of the Horus Heresy in the 31st Millennium the Warmaster Horus apparently only had 3 Primarchs and their respective Space Marine Legions at his side. Under his command were his own Sons of Horus, joined by Angron and his World Eaters, Fulgrim with the Emperor's Children, and Mortarion and the Death Guard. After ridding himself of all suspected Loyalist members inside the three Traitor Legions during the virus-bombing of Istvaan III, Horus chose the other habitable world in the Istvaan System, Istvaan V, as his command post and prepared a deadly trap for his Primarch brothers and their Loyalist Legions.

The Emperor of Mankind ordered the deployment of seven full Space Marine Legions against Horus and the known Traitor Legions, agonizing over the betrayal of his most beloved son. Unknown to the Emperor, four of the deployed Primarchs and their Legions had already turned against him and sworn themselves to the service of Chaos, forming a "fifth column" which would strike against the Loyalists at the most decisive moment.

Prior to the landing on the planet, Fulgrim and the Emperor's Children built fortifications and trenches, intended to protect the Traitor Legions from Loyalist orbital bombardment. The initial naval operations seemed to go well for the Loyalist side. The Imperial Navy managed to make orbit over Istvaan V and the Loyalist Legions proceeded with their planetary deployment. Thousands of Drop Pods and Stormbirds were deployed for the drop. The first wave was under the overall command of the Primarch Ferrus Manus and besides his own Legion, the Iron Hands, the Loyalist forces included the Salamanders led by Vulkan, and the Raven Guard under the command of their Primarch Corax. Vulkan's Legion assaulted the left flank of the Traitors, Ferrus Manus, Gabriel Santar, and 10 full companies of elite Terminators (called Morlock Terminators by the Iron Hands) went straight to the middle of the enemy lines, and Corax's Legion hit the right flank of the enemy position. The odds were considered equal; 30,000 Traitor Marines against 40,000 Loyalists. Horus was aware of the location of the Loyalists' chosen drop site and his troops fell upon the surprised Loyalist Legions. Ferrus Manus engaged Fulgrim in a ferocious duel, but ultimately the Primarch of the Iron Hands failed and was slain. The Loyalists retreated towards the apparent safety of the reserve force's lines.

Yet the Space Marine Legions of the reserve were no longer loyal to the Emperor, having already secretly sworn themselves to Chaos and the Warmaster. The Night Lords of Konrad Curze, the Iron Warriors of Perturabo, the Word Bearers of Lorgar, and the Alpha Legion of Alpharius fell upon their unsuspecting Loyalist brothers unexpectedly and the ensuing slaughter is widely known as the Drop Site Massacre of Istvaan V.

After the battle, the head of Ferrus Manus was delivered by Fulgrim to Horus as a trophy of the battle. Only a few Loyalist Space Marines, bearing the gene-seed of their fallen brothers and carrying the critically wounded Primarch Corax, managed to escape. A Loyalist Primarch had fallen in battle, and another was severely wounded. Still a third Loyalist Primarch, Vulkan, went missing in action, and how he ultimately managed to survive and to escape the Traitor Legions is still unknown. The outcome of the Drop Site Massacre was disastrous news for the Emperor and the Imperium. Due the developments at the world of Prospero the forces of the rebellion against the Emperor would be further strengthened by Magnus the Red and his Space Marine Legion, the Thousand Sons, who had become the unwitting servants of Tzeentch and had ultimately chosen to serve the Changer of Ways to protect their precious knowledge of sorcery. As the Heresy began in earnest, Horus possessed nine Space Marine Legions and had all but destroyed three of the remaining nine Loyalist ones. The path to Terra was wide open, and the decisive Battle of Terra and the Siege of the Imperial Palace would follow after seven more years of blood and terror as the Traitor Legions penetrated to the very heart of the Imperium of Man.

Contradictions
The system/planet's name is alternatively spelled Isstvan. An older version of this battle reports that initially the Emperor was quite reluctant to crush the revolt, as the reasons for the actions of the Warmaster were unknown. Fulgrim initially tried to reason and negotiate with his brother Primarch Horus only to be seduced by the power of Slaanesh.