Drop Site Massacre

The battle of Isstvan V, or the Drop Site Massacre as it would be better known, was one of the major defining battles of the Horus Heresy. After the revelation that Horus had betrayed him and the Imperium the Emperor sent the loyal primarchs and their legions to deal with the traitors. Ferrus Manus, Vulkan, and Corax, being the closest were sent as the vanguard of the Imperial force. Unknown to the loyalists fully half of their brother had betrayed him and the rest were simply waiting to openly declare their allegiance to Horus. After battling against the traitors the Iron Hands, Salamander, and Raven Guard were tired and exhausted, making their way back to their landing zone to resupply and coordinate with the other recently arrived "loyalists". As they neared the drop site the combined forces of the Iron Warriors, Night Lords, Word Bearers, and Alpha Legion opened fire on them revealing their true allegiance. As this happened the Sons of Horus, World Eaters, Emperor's Children, and Death Guard attacked from their rear completely encircling them. The loyalists were caught completely off guard and were slaughtered by the thousands during the fighting. Perhaps even worse was the lose of two of the legion's primarchs, as Ferrus Manus was killed by Fulgrim, and Vulkan disappeared. The loyalists were forced to perform an emergency extraction and barely managed to escape, however the three legions were crippled and largely unable to participate in the Heresy after the massacre.