>The loyalists commenced the attack by initiating a short orbital barrage all along the length of the traitor line. This proved almost totally ineffective due to the strength of the defensive system (as indeed the loyalists had earlier realised) but succeeded in throwing off the anti-air platforms long enough for an immediate massed drop pod assault to hit the Urgall Plateau directly in front of the traitor position.

>One account places the total amount of loyalist marines hitting the dirt in this assault as over 40,000 (See Notes). In one of the first pods to land, Ferrus Manus led his elite Iron Hands units directly into an incoming storm of gunfire. Dies Irae opened up, eliminating hundreds of loyalist marines in these first moments.

>Under cover of this mammoth weight of fire a unit of around a hundred traitor marines - made up of units from the Death Guard and Sons of Horus - sallied out to close and engage with Ferrus Manus' advance unit, but were quickly decimated and forced to retreat in the face of the primarch's rage. At roughly the same time the lead element of the Salamanders under Vulkan hit their portion of the enemy line. The traitors responded with a pinpoint artillery strike directly upon Vulkan's position... which barely fazed the Salamanders primarch, although it did slay several of his Firedrakes. With two primarchs penetrating the traitor defences and shrugging off everything thrown at them, the initial stage of the battle is considered to be tilted in the loyalists' favour.

>Further improving the loyalist situation, the support elements of the loyalists' first wave chose this time to land on the planet. Touching down in a pre-arranged landing zone at the other side of the Urgall Plateau from the traitor position, further loyalist Astartes forces moved out from their Thunderhawk and Stormbird transporters, while heavy landers beachd Imperial armour units (including Super Heavy Tanks) and artillery. With casualties taken into account, it's now reckoned about 60,000 Astartes are engaged in the battle.

>The traitor line bent like a bow under the weight of this attack, with Ferrus Manus' spearhead pushing in the furthest. At this point of wavering strength on the part of the defenders, Corax and the Raven Guard made their move, slicing into the traitor flank with a massed jump pack assault. However, this tactic was met with a riposte organised by Angron, primarch of the World Eaters, who had secreted many units of his legion in ambush positions, apparently for just this eventuality. His brutal warriors managed to slay many Raven Guard, halting the advance of the black-armoured Astartes. The loyalist push as a whole slowly ground to a halt at this point, as Mortarion stiffened the resolve of his Death Guard and Ezekyle Abaddon and Horus Aximand moved amongst the Sons of Horus, inspiring them by slaying any Imperial who got within their reach.

>Ferrus Manus' own constant forward movement finally ended when his Iron Hands ran directly into the waiting formations of Emperor's Children Noise Marines, who devastated the attacking Morlocks with their sonic weaponry.

>The Word Bearers, Iron Warriors, Night Lords and Alpha Legion executed successful combat landings into the already-established imperial drop zone, immediately fortifying it and securing the flanks of the plateau itself, with the Night Lords and Alpha Legion taking the flanks, the Iron Warriors the high ground behind the drop zone and the Word Bearers forming up on the newly erected defensive wall. The sight of this massive force - more than doubling the Imperial presence on Istvaan V in one stroke - appeared to force a general fighting retreat on the part of the traitorous forces, with even Angron, Mortarion and the Dies Irae seen pulling back from combat

>Just at this moment, Ferrus Manus located Fulgrim's command position in the centre of the traitor line and ordered his Morlocks to assault it, despite Corax's urgings to fall back. Corax believed that the battered first-wave Legions should take advantage of the lull in fighting to resupply in the drop zone encampment and return with the fresh second wave, and in fact both the Raven Guard and the Salamanders took this course of action. When Ferrus Manus refused to follow them, his two brothers apparently chose to leave him unsupported rather than reinforce his sudden forward push.

>The heavy Terminator elite of the Iron Hands struck the Emperor's Children command redoubt hard, engaging in battle with the significantly outnumbered Phoenix Guard. In their stead, Ferrus Manus confronted Fulgrim, choosing to duel him with words rather than weapons. At the other side of the battlefield, the Salamanders and Raven Guard, low on ammunition and having suffered heavy casualties, got to within a hundred metres of the landing zone fortifications when vox-contact with the second wave abruptly went dead. A single flare was fired from Horus' command post: a signal to the second wave legions, now revealed as traitors, to open fire. This first salvo decimated the unsuspecting Raven Guard and Salamanders.

>Holding an immediate conference, Corax and Vulkan found themselves disagreeing over what to do; Vulkan advocated making their way to their own dropships and digging in to resist an attack, while Corax insisted that they should take whatever means possible to immediately evacuate the area. Unable to agree a unified plan, Corax - realising the battle was lost - turned from his brother and ordered his legion to retreat by any means necessary.

>On the other side of the battlefield, the supposedly retreating traitor legions about-faced and threw themselves at the Iron Hands, apparently slaughtering them to the last marine. In the midst of this carnage, Fulgrim and Ferrus Manus, once the closest of brothers, dueled to the death. After a titanic conflict Fulgrim emerged the victor, and beheaded Ferrus Manus. Immediately afterwards, the horrified Primarch of the Emperor's Children, seeking oblivion for his sin, gave in to daemonic possession, for a time effectively ceasing to exist as an independent entity.

>The general advance of the traitors included the newly revealed second wave forces, with Lorgar, Kor Phaeron and Erebus of the Word Bearers in the vanguard. Due to the positioning of the legions, the Word Bearers primarily found themselves facing Raven Guard marines, and it was in the midst of this fighting that the hardest hitting units of both legions would meet in brutal combat. The Gal Vorbak - the Word Bearers' elite Possessed formation - leapt upon Corax, attempting to swarm him in close combat. The Primarch of the Raven Guard proved so formidable a warrior however, that even Astartes enhanced by daemonic possession were no match for him and he slew them freely. In an attempt to stop this slaughter of his favoured sons, Lorgar used his normally stunted and weak psychic powers to charge through the throng of warriors, arriving just in time to prevent the deaths of such Gal Vorbak as Argel Tal. Corax and Lorgar then dueled, with Corax swiftly gaining the upper hand over the less warlike Lorgar and preparing to execute him. Lorgar was only saved from death by the sudden intervention of Konrad Curze, the Night Haunter, who threw himself at his Raven Guard brother. Fresh to the battle and a lethal warrior, Curze proved superior to the tired and wounded Corax and drove him off.

>It was at this point in the battle that the day could truly be called a massacre. Massively outnumbered, the Raven Guard and Salamanders were dying, but dying slowly. Until that is, the return to the field of Mortarion, Angron and the Dies Irae, who caused tens and tens of deaths. Fulgrim - now secretly possessed by a daemon - quit the field completely, leaving the Emperor's Children to be commanded by Eidolon and Lucius. Finally, Horus himself entered the slaughter, leading his own Terminator elite; the Justaerin of Captain Falkus Kibre. At the climax of the massacre the Iron Warriors launched a tactical nuclear missile at Vulkan's position, annihilating those Salamanders with him and ending Vulkan's participation in the battle.