Isstvan V - 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 Isstvan III, Horus chose the other habitable world in the Isstvan System, Isstvan 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 Isstvan 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 Isstvan 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 after a tactical nuclear assault on his section of the battlefield and he was presumed dead.How he ultimately managed to survive and to escape the Traitor Legions remained largely unknown to the wider Imperium, though the fact that he was a Perpetual and thus capable of resurrection from almost any form of death now explains his unexpected survival.The outcome of the Drop Site Massacre was disastrous news for the Emperor and the Imperium. Due to the developments at the world of Prospero the forces of the rebellion against the Emperor were further strengthened by Magnus the Red and his 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 Siege of Terra would follow after seven more standard years of blood and terror as the Traitor Legions penetrated to the very heart of the Imperium of Man.