And so, the Siege of Terra began. For several weeks, the Traitor Legions and their allies hurled themselves at the walls of the Imperial Palace, and the Emperor's Children rampaged across the surface of the Throneworld, inflicting unspeakable horrors upon the population that the defenders of the Imperium had abandoned to focus on the Palace's protection.
On and on went the slaughter, Dorn's defenses proving a match for the Traitors' onslaught. The sun rose and fell behind clouds of dust and ash, and though millions died with every passing hour, the renegades were no closer to breaching the walls of the Imperial Palace and claiming victory. Eventually, astropaths on both sides of the conflict (those who hadn't been driven mad by the turmoil in the Warp as the Warmaster unleashed unspeakable horrors upon the Throneworld) began to hear a new distant voice in the chorus of wails : the echoes of the Ultramarines and the Dark Angels, whose fleets had long been delayed by trickery and strategy, about to reach Terra at last.
Yet the Imperial commanders withheld these news from their soldiers, save for a few leaders, for they believed that the defense of the Palace was only holding on by a thread, and that hope, in these circumstances, would be a deadly poison. Warriors who think they have nothing to lose fight harder than those who still hold the hope that they might survive the day after all. Similarly, among the traitors, Horus made sure to prevent knowledge of the approach of the First and Thirteenth Legion from spreading : his hold onto the forces under his banner was already fragile enough as it was. But the key players of the field knew, and that was enough.
In truth, for all that the Dark Gods' decision would alter the fate of the entire galaxy, the change that set it all in motion was a relatively small one. All it took was for the Four to bend their great power together in order to make sure that, when Horus, in a last gamble for victory, lowered the shields of the Vengeful Spirit and let his father board his flagship, the Warmaster did not cross the path of his brother, Sanguinius.
It was not an easy change. Sanguinius' doom at Horus' hands had long been written, witnessed and accepted by the Angel himself (though the Primarch was determined to make his death count, and his enemies pay for each drop of his blood). There was much resistance to this alteration of the plan, and powers that had been old before the Dark Gods had risen from the Warp's chaotic tides stood against them now, for they feared Sanguinius more than any of his brothers, still haunted by memories of Mankind's first tentative reaches toward the divine. The Dark Gods feared Sanguinius too : it was why they had gone to such lengths to corrupt him, and kill him when that had failed. But now they feared the golden shadow of the Eternal Tyrant more, and the old powers could not resist them for long.
And so Sanguinius wandered the corridors of the Vengeful Spirit, seeking the bridge and his fated confrontation with his brother. But the ship's interior twisted and changed around him, turning him around and around, and all his power and will could not break him free. Sons of Horus were guided toward him, and he slaughtered them all, for despite the wounds he had sustained defeating the daemon Ka'Bhanda, the Angel was more than a match for any number of Astartes.
Even the gifts bestowed by the Ruinous Powers upon some of the most worthy (or most foolish) sons of the Warmaster were not enough. Only Horus could kill him, for the Dark Gods had not unwritten his demise at the hands of their champion : they had merely delayed it, pushed it back in the distant future. Even they did not know what the full consequences of such an act would be. Perhaps Kairos Fateweaver, the Oracle of Tzeentch, knew, as it knew most things past and future. But since the gathering of the Four in the ruined palace of their unborn brother, all the Oracle had done was laugh in the Court of its master, its two heads shrieking and gasping endlessly.
While the Angel was kept away from his doom by the machinations of the Dark Gods, the Emperor tore a path toward the bridge of the ship, seeking His treacherous progeny. Nothing the armies of the Arch-Traitor threw at Him could hurt Him, but one by one, the Custodes and Imperial Fists who had accompanied Him in the assault fell or were separated from Him. And so, when He finally faced Horus on the bridge of the Vengeful Spirit, the Master of Mankind was alone. What words they exchanged, if any, were lost to posterity. And what did words mean, after seven years of a war that had set the galaxy ablaze and left trillions dead ?
In a single moment, the Talon of Horus pierced through the Emperor's heart, and His sword cut through the Warmaster's armor and deep into his flesh. Father and son fell to the deck, their blood mingling onto the metal floor, the vitae igniting where the psychically charged blood of the Emperor met the Chaos-tainted blood of His son.
As the Sons of Horus watched in horrified silence as their father fell, an old, human man appeared on the bridge, emerging from the shadows. He was named Ollanius Persson, and he had come far to be here – but it seemed that he had been too late. He could have struck Horus down there and then : the Warmaster was bleeding, unconscious, and Persson was far more dangerous than he appeared. But if he did that, then the Space Marines in the room would kill him, and the Emperor would die too, and the consequences of that were too grave to dwell upon. So he did the only thing he could think of : he hauled the Master of Mankind's dying form onto his shoulders (for despite all His power, in the end, the Emperor's body was only that of a man) and vanished again, cutting a hole into reality with a blade he had taken from a dead Word Bearer on distant Calth.
Sanguinius felt his father's and brother's fall and the former's disappearance from the ship and sudden reappearance on the surface of Terra, and he knew that he had failed : somehow his doom had been adverted, at a terrible cost. The Angel screamed in impotent rage, and the echoes of his fury would haunt the corridors of the Vengeful Spirit forevermore, spawning winged, black wraiths that would prey upon the ship's crew and leave them drained of blood.
The Primarch of the Blood Angels calmed quickly, however, and immediately began to make his way out of the ship. He called to the rest of his sons and the other loyalists scattered across the vessel, and they came together, fighting their way toward one of the flight deck, where they seized a handful of gunships (there were few enough of them left that they didn't need many of them) and flew out of the Vengeful Spirit, broadcasting identity codes to ensure they weren't shut down by Imperial guns. They flew straight for the Imperial Palace, where Sanguinius could sense the quickly fading presence of his father.
Meanwhile, aboard the Vengeful Spirit, the anarchy was almost total. Horus still lived, but he had been grievously injured, even more severely than when Russ had struck him with his accursed Spear. Back then, it had taken Maloghurst's strange ritual to rouse the Primarch; this wound was even deeper, but at least there didn't appear to be any sorcery attached to it : merely the raw, burning power of the Emperor's blade. They rushed their master to the nearest Apothecarion, but a few cooler heads remembered that the First and Thirteenth Legions would soon be here. With Horus incapacitated, there was no one who could unify the renegade Legions and finish the war before the arrival of Guilliman and the Lion.
So they ran. The surviving members of the Mournival called all of their brothers and support troops on the surface to withdraw, sending messages to the other Legions and rebel forces to do the same. They didn't wait to see if they were obeyed, nor did they listen to the outraged replies of their allies. The Sons of Horus made for the edge of the system, and as they did so, the psykers among them received a vision : they saw that their father could be healed, if they took him to the Eye of Terror. Considering the reports coming from the Apothecarion, that was about the only hope the Sixteenth Legion had, and so they set course for the Occulis Terribilis. Piece by piece, the rest of the traitor armada broke off from the Siege, their entire formation collapsing without any grace. Thousands of Traitor Marines and millions of human soldiers perished who could have lived if the retreat had proceeded in good order.
As the traitors fled, Dorn found the old man carrying his father's bleeding body. Ollanius vanished before the Praetorian could say or do anything, and Dorn brought the Emperor to the Golden Throne, where the dust that had been Malcador the Sigillite still sat, undisturbed. In desperation, he placed the Master of Mankind's dying form upon the Throne, and activated the stasis field, trapping his father between life and death and connecting Him to the great psychic fire of the Astronomican.
Elsewhere in the Sol system, the moon of Saturn that held the first of the Grey Knights returned to the normal flow of time, and the warriors within the stronghold, who had spent subjective decades learning all the lore left to them by Malcador and mastering their new abilities, immediately sensed that something had gone horribly wrong in their absence. Their orders demanded that they kept themselves concealed even from the loyal Primarchs, but they sent agents to learn what had transpired in their absence, while consulting their tools of divination to learn more.
And then, at last, Kairos stopped laughing, and vanished from the Court of Change, gone to perform its craft of deception and scheming elsewhere. The two-headed Daemon Lord went into the Eye of Terror, to a world that was yet nameless. With a gesture, it shaped the earth into a citadel, and sat upon a throne at the heart of that citadel. Then it waited, knowing what must come next.