Raven Guard - The Horus Heresy: During the Horus Heresy, the Raven Guard were involved in the Drop Site Massacre of Istvaan V, where four Space Marine Legions, including the Night Lords, Iron Warriors, Word Bearers and the Alpha Legion, turned to Chaos under the banner of the Arch-Traitor Horus and unexpectedly betrayed their Loyalist Battle-Brothers, catching them off-guard in a terrible cross-fire.The Raven Guard fought ferociously alongside their fellow Loyalists of the Salamanders and the Iron Hands Space Marines against the Traitor Marines of Chaos, but suffered such heavy losses that they were effectively crippled and unable to play any further constructive role in the defence of Terra from Horus' assault during the Heresy.The Raven Guard had been all but annihilated at the Drop Site Massacre of Istvaan V, reducing the XIXth Legion's strength from some 80,000 Astartes to less than 3,000 survivors.Taking stock of their dire situation, the Primarchs Corax and Vulkan differed over how to salvage what they could from the situation. The Salamanders' Primarch suggested that the Loyalists attempt to make a tactical withdrawal to their respective drop ships and dig in to resist any further attacks.Corax advocated instead that the Loyalists should do whatever they could to make good their escape from the slaughter as the battle was lost. Neither Primarch could agree with the other, and so Corax turned from Vulkan and ordered his Legion to retreat.A short while later, a direct artillery strike hit the Primarchs' position. By the grace of the Emperor, Corax somehow managed to survive, but as to the fate of his brother Vulkan, nothing at the time was known.In the meantime, the second wave of Traitor Legions turned upon their former allies and began to slaughter them in earnest. The Iron Hands were apparently slaughtered to a man.In the midst of this brutal carnage two Primarchs from the opposing sides confronted one another in a final showdown. Fulgrim of the Emperor's Children and Ferrus Manus, once as close as true brothers could be, fought a bitterly contested, titanic struggle. In the course of the heated duel Ferrus Manus lost and was beheaded by his former brother.On the other end of the battlefield, the Raven Guard were confronted by the treacherous Word Bearers, with their Primarch Lorgar, the First Captain Kor Phaeron and the First Chaplain Erebus at their vanguard. The two Legions fought one another in bitter combat.In the midst of this battle, the Word Bearers unleashed the elite unit known as the Gal Vorbak -- Astartes who had allowed themselves to be possessed by daemons. They attacked the Raven Guard's Primarch en masse, but despite the advantage of their numbers, Corax's formidable abilities as a consummate warrior proved to be more than a match for the possessed Astartes, and he slew them with impunity.Seeing the slaughter of his most favoured sons, Lorgar intervened and prevented the death of the remaining Gal Vorbak. The two opposing Primarchs duelled one another in close combat, and the Raven Guard's Primarch quickly gained the upper hand over his outmatched brother.Lorgar had always been more of a scholar than a warrior and Corax prepared to execute him for his betrayal of the Emperor. Lorgar was spared from execution by the intervention of the Night Lords' Primarch, Konrad Curze, at the last moment.The Night Haunter and the Raven fought a brutal melee. Curze quickly gained the upper hand over his battle-weary brother and prepared to slay him, but Corax managed to escape death by taking to the sky with his master-crafted Jump Pack.The outnumbered Loyalists were then surrounded and brutally butchered. Refusing to surrender, the remaining Raven Guard and Salamanders Astartes stubbornly defended themselves, trying to hold off the inevitable slaughter for as long as possible.Though they suffered an atrocious number of casualties, the Loyalists managed to hold their own, until the Primarchs Mortarion of the Death Guard and Angron of the World Eaters joined the fray.Bolstered by the support of the infamous Imperator-class Titan Dies Irae, the Traitors killed tens of thousands of Loyalist Astartes.At the height of the massacre the Warmaster Horus entered the combat, at the head of the elite Sons of Horus Terminators known as the Justaerin, slaughtering the Loyalists in wrathful anger.Any hope of escape for the Loyalists was quickly crushed when the traitorous Iron Warriors destroyed the first wave's drop ships. The Loyalist starships still orbiting the embattled planet were also largely annihilated by the vastly superior numbers of the Traitors' fleet.Despite the odds arrayed against them, some of the Loyalists on the ground managed to survive -- they miraculously escaped through the tightening cordon of Traitors that surrounded their position. The Raven Guard fared better than the Salamanders in escaping the brutal massacre. But the Salamanders managed to assist a few surviving Astartes from the decimated Iron Hands Legion to also escape the slaughter.Imperial history does not record the fate of these surviving Salamanders or their missing Primarch Vulkan. The Raven Guard's Primarch just barely managed to board a fleeing Thunderhawk gunship to make good his escape, but was thwarted in the attempt when it was shot down almost immediately by the gunfire of the Traitors. The badly damaged gunship crashed on the outskirts of the Urgall Plateau.