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Isstvan V was the site of the near demise of the [[Iron Hands]], [[Salamanders]] and [[Raven Guard]] loyalist [[Space Marine]] [[Space Marine Legion|Legions]] following a trap set by [[Horus]]. The plan essentially consisted of opposing the loyalist marines with the openly [[Traitor Legion|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.{{Cite thisMarker End}}

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.{{Cite thisMarker End}}

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 to gain orbit over Isstvan V and the Legions proceeded with their planetary deployment. Thousands of [[Drop Pods]] and [[Stormbird]]s 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 [[Space Marine Company|companies]] of [[Morlocks|Morlock Terminators]] 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.{{Cite thisMarker End}}

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 summarise the whole battle: '''"When the traitor's hand strikes, it strikes with the strength of a Legion."'''{{Cite thisMarker End}}

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.{{Cite thisMarker End}}

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.{{Cite thisMarker End}}

*{{Endn|2}}: [[Realm of Chaos: Slaves to Darkness]], {{Cite ThisMarker End}}

*{{Endn|4}}: [[Fallen Angels (Novel)]], {{Cite ThisMarker End}}