In my version of the tale, Horus was sent to cleanse the Istvaan system. On the voyage, Mortarion sought council with him, but instead poured the machinations of the Chaos Gods into his ear. Knowing of Horus' position as Warmaster, Mortarion reasoned that he could swerve more legions to his cause than some of the other Primarchs. Horus called a meeting of all nearby Primarchs, who happened to be Magnus, Lorgar, Curze and the noble Sanguinus.
Horus told them of the great powers to be theirs, and each aligned themselves with one of the four. Magnus, who had forever spent his time trying to gain knowledge on Prospero, decided that he deserved a pleasurable vacation and fell into the embrace of the Dark Prince. Curze, who had ever been plagued with visions, sided himself with Tzeentch in an effort to stop these nightmares.
Sanguinus was at first reluctant, but found that in close up combat, the ability of pain immunity proved to great a lure and he became the champion of Grandfather Nurgle. Finally, Lorgar discovered that his rituals had ever been watched by Khorne, and the great Cathedrals of the Word Bearers became bloated on depraved Blood Sacrifices.
Gradually, the word spread from Primarch to Primarch that Horus and his lieutenants had been corrupted. For Rogal Dorn, Corax and the Lion El'Johnson, the unnatural powers of chaos swayed them and the attack on Terra was formulated...
Rogal Dorn knew the Terran defences inside out, and he alone knew of a secret exit, built if the Emperor was ever in peril. A small force of the Sons of Horus snuck into this entrance as the mighty batteries of Perturabo and Dorn battered the palace walls. This strike team was eliminated by Fulgrim and Guilliman, who had been organizing a counter attack before they stumbled across the traitors. This was the gauntlet thrown.
The Emperor and his loyal sons battled across the plains bordering the Imperial Palace, a force to be reckoned with as they strode through the mightiest of bombardments without harm. Eventually they came across the five original traitors atop a great rocky plateau; Magnus, now lightning fast and sweating an alluring perfume, was cut down by Leman Russ, but no sooner was the Red Witch dead when Sanguinus appeared. The Great Wolf slashed and hacked at the Vampire but to no avail. With a great sword dripping with filth, Sanguinus beheaded Russ, intestines slopping over his former brother. As the battle went on, and the remaining Primarchs were cut down, no one stopped to see the duel between Mortarion and the Emperor. The Death Guard Primarch unleashed foul noxious fumes that had been brought from his home planet, but he Emperor's armour dispelled the vile gases.
As Mortarion forced the Emperor back, the Master of Man saw a great opportunity. He feigned weakness, falling backwards. As the fallen Primarch stood over him, ready to bring his scythe cleaving downwards, the Emperor threw his great sword at the gate mechanism. The spiked golden portcullis thundered down, biting into Mortarion. As he fell to the ground, he wept, and a great darkness was lifted from his eyes. As he slowly bled to death, he whispered 'O Father, what have I done?'. The Emperor desperately tried to lift the gate, seeing now that it was not his son who had done this terrible thing. The gates were strong, and the Emperor could not lift them. He collapsed, sobbing, knowing that he had killed his own son. The forces of chaos fled in disarray.
When the other Loyalists found him, they hijacked the gate mechanism and opened it before leaving the Emperor with his son's almost lifeless body. As Mortarion slowly died, the Imperator tried to cover the wounds he had made with his hands, but damage was too great, and his hands too small. He decided he could not live with what he had done, so sat upon the Golden Throne, where he still resides to this day, desperately searching the spirit plane for the soul of his dead son.
Last edited by MidnightSun; 06-03-10 at 08:36 AM.