Only in death could this end.
The retreat of the Warmaster's forces appeared ragged and faltering, exactly as Horus had planned it. Warriors streamed back from the front lines of battle in determined groups, their spirits apparently broken, but gathering in knots of resistance behind shelled ruins and fire-blackened craters.
The Iron Hands pushed through the defences, the bulky Terminators unstoppable in their relentless advance. Lightning crackled from the claws of their gauntlets and their red eyes shone with anger. The Phoenix Guard braced themselves to meet the charge, fully aware of the power of such mighty suits of armour.
Marius released a howl of ecstatic joy, and his bizarre weapon amplified it into a screeching wail of deadly harmonics that ripped through the ground in a roaring sonic wave to explode amongst the front ranks of the Morlocks.
The giant warriors were torn apart in a clashing shriek of aural power as the apocalyptic noise made play of their armour and butter of their flesh. The Emperor's Children screamed in pleasure at the sound, their enhanced senses and augmented brain paths rendering the discordant sounds into the most vivid sensations imaginable.
'When they come,' shouted Fulgrim, 'leave Ferrus Manus to me!'
The Phoenix Guard answered with a terrible war cry and leapt to meet the Morlocks in a searing clash of blades. Electric fire leapt from the golden edges of the halberds and claws of the warriors, and a storm of light and sound flared from each life and death struggle. The battle engulfed the Primarch of the Emperor's Children, but he stood above it, awaiting the dark armoured giant who strode inviolate through the lightning shot carnage as brothers hacked at one another in hatred.
Fulgrim nodded in greeting as Ferrus reached towards a sword belted at his waist, and he smiled as he recognised Fireblade's hilt.
'You remade my sword,' said Fulgrim, his voice cutting through the atrocious din of fighting. Though the ferocious battle between the Morlocks and the Phoenix Guard surrounded them, neither primarch's praetorians dared approach them, as though aware that to transgress this fateful confrontation would be a heinous crime.
'Only to see you dead by a weapon forged by my own hand,' spat Ferrus.
In response, Fulgrim sheathed his silver sword and reached behind him to unlimber the great warhammer held at his back. 'Then I shall do likewise.'
The great weight of Forgebreaker, the weapon his own skill and energies had crafted beneath the peaks of Mount Narodnya, felt good in his hands as he descended the rock to face his erstwhile brother.
'It is fitting we face one another with the weapons we forged long ago,' said Fulgrim.
'I have long waited for this moment, Fulgrim,' replied Ferrus, 'ever since you came to me with betrayal in your heart. For months I have dreamt of this reckoning. Only one of us will walk away from this, you know that.'
'I know that,' agreed Fulgrim.
'You betrayed the Emperor and you betrayed me,' said Ferrus, and Fulgrim was surprised to hear genuine emotion in his brother's voice.
'I came to you because of our friendship, not despite it,' answered Fulgrim. 'The universe is changing, the old order upset and a new dawn approaching. I offered you the chance to be part of the new order, but you threw it back at me.'
'You sought to make me a traitor!' snarled Ferrus. 'Horus is mad. Look at all this death! How can this be right? You will hang from Traitor's Gibbet for this sedition, for I am the Emperor's loyal servant and through me his will and vengeance will be done.'
'The Emperor is a spent force,' snapped Fulgrim. 'Even now he whittles away on some trivia in the dungeons of Terra while his realm is in flames. Are those the actions of a being fit to rule the galaxy?'
'Do not think you can win me to your cause, Fulgrim. You failed once and you will not get a second chance.'
Fulgrim shook his head. 'I am not offering you a second chance, Ferrus. It is already too late for you and your warriors.'
Ferrus laughed at him, but he could sense the despair in it. 'Are you mad, Fulgrim? It's over. You and the Warmaster are defeated. Your forces are routed and the power of another four Legions will soon crush your attempt at rebellion utterly.'
Fulgrim was unable to keep the sensations seething in his head contained any longer and he shook his head as he savoured his next words. 'My brother, how naive you are. Do you really think Horus would be foolish enough to trap himself like this? Look to the north and you will see that it is you who are undone.'
The forces of the Raven Guard and Salamanders fell back in good order to the drop zone, where their reinforcements were deploying to join the fight. The drop-ships of the Iron Warriors, armoured bastions connected by high walls of spiked barricades, formed an unbroken line of grim fortifications on the northern slopes of the Urgall Depression.
A force larger than that which had first begun the assault on Isstvan mustered in the landing zone, armed and ready for battle, unbloodied and fresh.
Corax and Vulkan led their forces back to regroup and to allow the warriors of their brother primarchs a measure of the glory in defeating Horus, dragging their wounded and dead with them. The victory had been won, but the cost had been steep indeed, with thousands of all three Legions lost to the betrayal of the Warmaster. Horus's forces were in retreat, but there would be no celebration of the slaughter, no joyous victory feasts or glorious days of remembrance, only another sad scroll added to a banner that would never again see the light of day.
Scorched tanks rumbled alongside the Astartes, their ammunition expended and their hulls battered by the impact of shot and shell.
Unanswered vox hails requested medical aid and supply, but the line of Astartes at the top of the north ridge was grimly silent as the exhausted warriors of the Raven Guard and Salamanders came to within a hundred metres of their allies.
A lone flare shot skyward from inside the black fortress where Horus had made his lair, exploding in a hellish red glow that lit the battlefield below like a madman's vision of the end of the world.
And the fire of betrayal roared from the barrels of a thousand guns.
Fulgrim laughed at the stunned look on Ferrus's face as the forces of his ''allies'' opened fire upon the Salamanders and Raven Guard. Hundreds died in the fury of the first moments, hundreds more in the seconds following, as volley after volley of bolter fire and missiles scythed through their unsuspecting ranks. Explosions flashed to life in their midst, vaporising warriors and tearing through tanks as the force of four Legions ripped the beating heart from the first wave of loyalists.
Ferrus Manus watched in mute horror as he saw a storm of fire engulf Corax, and a titanic explosion mushroom skyward from where Vulkan stood in astonished outrage at what was happening.
Even as terrifying carnage was being wreaked upon the loyalists below, the retreating forces of the Warmaster turned and brought their weapons to bear on the enemy warriors within their midst. Hundreds of World Eaters, Sons of Horus and the Death Guard fell upon the veteran companies of the Iron Hands, and though the warriors of the X Legion continued to fight gallantly, they were hopelessly outnumbered and would soon be hacked to pieces.
Ferrus Manus turned to face Fulgrim, and the Primarch of the Emperor's Children could see the despair etched into his brother's features, his silver eyes dull and lifeless. To have so great a victory snatched away in an instant must be the most sublime sensation. Fulgrim almost wished to switch places with his brother just to taste that feeling for himself.