The planet of Isstvan V is widely known for the infamous Drop Site Massacre.
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Isstvan V was the site of the near demise of the Iron Hands, Salamanders and Raven Guard loyalist Space Marine Legions following a trap set by Horus. The plan essentially consisted of opposing the loyalist marines with the openly traitorous legions the Sons of Horus, World Eaters, Death Guard and Emperor's Children; followed by having the Night Lords, Iron Warriors, Word Bearers and Alpha Legion, who were believed to be loyal but were actually pledged to Horus, stab the loyalists in the back. It resulted in the almost complete annihilation of the three loyalist legions, with only a few battle brothers escaping alive, and left the Imperium severely weakened in the face of Horus' onslaught.
At the start of the Horus Heresy, Warmaster Horus apparently only had three other Primarchs and their respective Legions at his side. Under his command were his own Sons of Horus, joined by Angron and his World Eaters, Fulgrim with his Emperor's Children, and Mortarion and the Death Guard. After ridding himself of all suspected Loyalist members inside the three Legions at Isstvan III, he choose Isstvan V as his command post and prepared a deadly trap for his former brothers and their Legions. The Emperor ordered the deployment of seven full Space Marine Legions against him, agonizing over the betrayal of his most beloved son. Unknown to the Emperor of Mankind, four of the deployed Primarchs and their Legions had already turned against him, forming a "fifth column" which would strike against the Loyalists at the most decisive moment.
Prior to the landing of the planet, Fulgrim and the Emperor's Children built fortifications and trenches, able to protect the traitor legions from orbital bombardment. The initial naval operations seemed to go well for the Loyalist side. The Imperial Navy managed gain orbit over Isstvan V and the 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 Ferrus Manus and besides his own Legion, the Iron Hands, the Loyalist forces included the Salamanders led by Vulkan, and the Raven Guard under Corax. Vulkan's Legion hit the left side of the traitors, Ferrus Manus, Gabriel Santar, and 10 full companies of terminators(morlocks) went straight to the middle, and Corax's Legion hit the right side of the enemy position. They landed right into a bloodbath. The odds were considered equal; 30,000 traitors against 40,000 loyalists. Horus was aware of the chosen drop site and his troops fell upon the surprised Legions. Ferrus Manus engaged Fulgrim in a ferocious duel, but ultimately failed and was defeated. The Loyalists retreated towards the apparent safety of their brothers of the second wave.
The Legions of the second wave were no longer loyal to the Emperor. 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 brothers and the ensuing slaughter is widely known as the Drop Site Massacre. A phrase from the Warmaster himself can easily resume the whole battle: "When the traitor's hand strikes, it strikes with the strength of a Legion."
After the battle, the head of Ferrus Manus was delivered by Fulgrim to Horus as a trophy. Only a few Loyalist Space Marines, bearing the gene-seed of their fallen brothers and carrying the critically wounded Corax, managed to escape. A loyal Primarch had fallen in battle, another was severely wounded, and a third was missing in action. Vulkan was missing, and how he ultimately managed to survive and to escape is quite unclear. It was disastrous news for the Emperor and the Imperium.
Due to the developments at Prospero the rebellion would be further strengthened by Magnus the Red and his Legion, the Thousand Sons, servants of Tzeentch. Horus had nine Space Marine Legions and had all but destroyed three loyal ones. The way to Terra was wide open, and the Battle of Terra would follow.
The system/planet's name is alternatively spelled Istvaan.
An older version of this battle reports that initially the Emperor was quite reluctant to crush the revolt, as the reasons of the Warmaster were unknown. Fulgrim initially tried to reason and negotiate with the Warmaster only to be seduced by the powers of Slaanesh.
- 1: Renegades, page 61, by Andy Chambers, Jervis Johnson & Rick Priestley
- 2: Realm of Chaos: Slaves to Darkness, [Needs Citation]
- 3: Codex: Chaos Space Marines (3rd Edition, 2nd Codex), pg. 5
- 4: Fallen Angels (Novel) by Mike Lee, [Needs Citation]
- 5: The Horus Heresy Book Two - Massacre, page 26 (sidebar), by Alan Bligh
- 6: The Horus Heresy Book One - Betrayal, pg. 1