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The Agonizing over the betrayal of his most beloved son, the Emperor ordered the deployment of seven full Space Marine Legions against him, agonizing over the betrayal of his most beloved son: their . Their orders were to take Horus and the Primarchs allied with him into custody, then and bring them back to Terra to explain their actions. Unknown Unbeknownst to the Emperor of Mankindhowever, four of the deployed Primarchs and their Legions chosen for this task had already turned against him, forming a "fifth column" which would strike against the Loyalists at the most decisive moment.

The initial naval operations seemed to go well for the Loyalist sideLoyalists. The Imperial Navy managed gain gained orbit over Isstvan V and the Legions proceeded with their planetary deployment.

Three whole Legions took part in the first wave of landings, under Under the overall command of [[Ferrus Manus]] and besides his own Legion, three whole Legions took part in the [[Iron Hands]], included first wave of landings; the [[Salamanders]] lead by [[Vulkan]], and the [[Raven Guard]] under [[Corax]] and Manus's own Legion the [[Iron Hands]]. Horus had foreknowledge of the location of the loyalist drop site, and his forces mauled the loyalist Legions during their landings, keeping them pinned down at the drop site and unable to advance on the foe.

Ferrus Manus engaged Fulgrimin personal combat, only to die at his hands, while the Emperor's Children butchered the veterans of the Iron Hands. The Loyalists retreated towards the apparent safety of their brothers of in the second wave, hoping to gain reinforcements: reinforcement; what happened next took them completely by surprise and went down in infamy.

The four Legions of the second wave - The [[Night Lords]] of [[Konrad Curze]], the [[Iron Warriors]] of [[Perturabo]], the [[Word Bearers]] of [[Lorgar]], and the [[Alpha Legion]] of [[Alpharius]] - already seduced to the cause of Horus, instead of attacking the rebels, opened fire, slaughtering on their unsuspecting brothers in a , slaughtering them wholesale. This orgy of carnage would later become widely known as the ''Istvaan V Drop Site Massacre''.

A phrase from the Warmaster himself can easily resume sumarise the whole entire 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 presented by Fulgrim to Horus as a trophy. It is widely believed that Horus planned the battle with aid from the tactical genius, Alpharius.

Only five Loyalist Space Marines, bearing the gene-seed of their fallen brothers and carrying the critically wounded Corax, managed to escape. They fled back to the Raven Guard's homeworld, Deliverance , to tend to their wounds and alert the Emperor what had happened.

In a single stroke, Horus had destroyed three entire loyalist Legions. A loyal , a 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 still quite unclear. It was disastrous news for the Emperor and the Imperium.

During the developments at [[Prospero]] the , Horus's rebellion would be further be strengthened by [[Magnus the Red]] and his Legion, the [[Thousand Sons]], now servants of [[Tzeentch]]. With nine Legions and much of the Adeptus Mechanicus behind him, Horus quickly struck towards Terra.²