Event · M31
Isstvan V Betrayal
The Isstvan V Betrayal was the ambush of loyalist Space Marine Legions by Horus’s forces on Isstvan V in 007.M31, which began open war in the Horus Heresy. It is also known as the Dropsite Massacre, after the landing zones where the loyalist Legions were caught. It followed the earlier action on Isstvan III, where the Warmaster, already turned to Chaos, used a colonial revolt as a pretext to separate the disloyal element of his own Legions from those still faithful to the Emperor. The Sons of Horus, Emperor’s Children, Death Guard and World Eaters took part in that purge, and it left the traitor cause committed to a course it could no longer conceal. Isstvan V lay in the same system. The Imperium’s response to the first reports sent the Iron Hands under Ferrus Manus, the Raven Guard under Corax and the Salamanders to the surface. They expected to reinforce a campaign against a rebel force that had already been weakened. On landing they found the Warmaster’s Legions, with allied Titan Legions and auxiliary forces, in strength and prepared. Accounts record that the loyalist drop sites came under concentrated fire almost at once, and that Astartes within the traitor Legions who remained loyal were among the first to be killed. Ferrus Manus is recorded as having been killed by Fulgrim, primarch of the Emperor’s Children, during the fighting. The loyalist Legions suffered losses on a scale that removed most of their fighting strength. Corax withdrew with a small remnant of the Raven Guard, and survivors of the Iron Hands and Salamanders were scattered or evacuated in the days that followed. The action ended any uncertainty about Horus’s allegiance and left the Imperium with far fewer loyalist forces to oppose him. Later tradition treats it as the point after which the Heresy became an open contest for Terra, culminating in the Siege of Terra in the closing years of M31. On the Orrery map it is anchored to Isstvan V, with the Isstvan III atrocity as its direct precursor on the timeline.
See where Isstvan V Betrayal occurred on the galactic map during the Horus Heresy.