Drop Site Massacre - Initial Assault: The warriors of the World Eaters, Death Guard, Sons of Horus and Emperor's Children had deployed throughout the defences constructed along the ridge of the Urgall Depression, making ready for the howling storm of battle that was soon to descend upon them. Behind them, long range support squads manned the walls of the fortress, and Traitor Army artillery pieces waited to shower any attacker with high explosive death. The Dies Irae stood before the wall, its colossal guns primed and ready to visit destruction on the enemies of the Warmaster. Hundreds of thousands Astartes hunkered down on the northern edge of the Urgall, their guns ready and their hearts steeled to the necessity of what must come.The first warning came when a dull, red orange glow built behind the clouds, bathing the Urgall in a fiery light. Then came the sound: a low roar that built from a deep, thrumming bass to a shrieking whine. Alarms sounded and the clouds split apart as individual streaks of light burned through and fell in a cascading torrent of fire. Thunderous explosions ripped along the edge of the Urgall, and the entire length of the Warmaster's forces was engulfed in a searing, roaring bombardment. For long solar minutes, the forces of the Emperor that had just moved into orbit over Istvaan V pounded the Urgall Plateau from orbit, a firestorm of unimaginable ferocity hammering the surface of Istvaan V with the power of the world's end.Eventually, the horrific bombardment ceased and the drifting echoes of its power faded, along with the acrid smoke of explosions, but the Emperor's Children had performed perfectly in creating a network of defences from which to face their former brothers, and the forces of the Warmaster had been well-protected. From his vantage point in the alien-built keep, Horus smiled, and he watched the sky darken once again as thousands upon thousands of Drop Pods and Stormbirds streaked through the atmosphere towards the planet’s surface to carry out the initial Loyalist assault.The first wave was under the overall command of the Primarch Ferrus Manus and besides his own X Legion, the Salamanders led by Vulkan and the Raven Guard under the command of their Primarch Corax joined him. Vulkan's Legion assaulted the left flank of the Traitors' battle line while Ferrus Manus, the Iron Hands' First Captain Gabriel Santor, and 10 full companies of elite Morlocks Terminators charged straight into the centre of the enemy lines. Meanwhile, Corax's Legion hit the right flank of the enemy's position. The odds were considered equal. Horus was aware of the location of the Loyalists' chosen drop site and his troops fell upon the Loyalist Legions.The battlefield of Istvaan V was a slaughterhouse of epic proportions. Treacherous warriors twisted by hatred fought their former brothers-in-arms in a conflict unparalleled in its bitterness. The mighty Titan war engines of the Machine God walked the planet's surface and death followed in their wake. The blood of heroes and traitors flowed in rivers, and the hooded Heretek Adepts of the Dark Mechanicum unleashed perversions of ancient technology stolen from the Auretian Technocracy to wreak bloody havoc amongst the Loyalists.Hundreds, perhaps even thousands of Traitors had been slaughtered in the opening moments of the assault. All across the Urgall Depression, hundreds died with every passing second, the promise of inevitable death a pall of darkness that hung over every warrior. Thousands were dying every minute, the slaughter terrible to behold. Blood ran in rivers down the slopes of the Urgall Depression, carving thick, sticky runnels in the dark sand. Such destruction had never yet been concentrated in such a horrifically confined space, enough martial power to conquer an entire planetary system having been unleashed in a line less than twenty kilometres wide.The slaughter continued unabated, on a scale never before seen, with neither side able to press home their advantages. The Traitors were well dug-in and had defensible positions, but the Loyalists had landed almost directly on top of them with numerical superiority. The bloodletting was a truly horrific sight as warriors who had once sworn great oaths of loyalty to one another fought their brothers with nothing but hatred in their hearts. No Legion fared well in the slaughter, as the scale of the fighting rendered tactics meaningless as the two armies battered each other bloody in a remorseless conflict that threatened to destroy them all.The Traitor forces held, but their line was bending beneath the fury of the first Loyalist assault. It would take only the smallest twists of fate for it to break. The forces on the surface were in combat for almost three hours with no clear victor emerging. The Loyalists waited for the second wave of "allies" to make planetfall, believing they would be reinforced for their final advance. The Traitors all knew the parts they had to play in this deadly performance. They were all aware of the blood they needed to shed to install Horus as the Master of Mankind.