Location: The Heart of the Inner Sanctum, Luna
Date: 813.M30 (Thirteen hours since the battle for Luna began)
It was Evil Incarnate. That was the only way Abaddon could describe the nightmarish being before him. Nearly every human civilization had some myth or concept of the Adversary. Satan, Lucifer, Apophis, Set, Erllig, Iblis, Angra Mainyu, Be'lakor, Ba'al, Azura, Nyarlathotep, and many many more. This idea, the idea of the primordial enemy, evil given form and purpose, it had gained sentience and power within the Warp. The thoughts of all of those thinking creatures had coalesced into a single being of malice and horror. It formed into the Dark Master.
Now Be'lakor, the First Damned of Chaos, had entered the material universe. Empowered and given body by the Creed of Four Phases, the Daemon Prince's fanged maw split into a grin as it noticed the fallen Primarch. Moving ponderously, as it grew used to the rapidly fading laws of physics. It moved with an almost apathetic gait. The universe seemed to ripple and contort as it walked towards the Astartes, as if space/time itself fought to move from his infernal presence.
The Daemon Prince spoke in a deep and eloquent tone. "I had hoped to claim the XVI as my host" the creature said with a rueful glance at the fallen hero. "He would have made a perfect body. The Dark Gods would never possess a mightier Agent. Alas, this sorcerer will do."
Shadowy warp-fire coalesced around Be'lakor 's claws. Approaching Horus, it let out a mirthless laugh. "Still, a body such as that has so many uses."
Abaddon was quick to realize the monster's intent. It wished to desecrate his father's body. He would not allow the fiend to do as it pleased. To Abbadon, the honor of the entire XVI Legion was at stake. Standing between Primarch and Prince, Abaddon intended to die stopping the monster. Some part of the young Astartes knew this is exactly where he belonged, standing before Evil Incarnate and guarding mankind's destiny. He had done so aboard the Tiber-Prince, and he would continue to do so until his last breath.
Be'lakor stood twice the height of a Primarch and peered down at the Astartes before him. "Oh? Another Warmaster approaches me. The First failed to embrace the gods. Will you fail yet again Despoiler?"
Roaring with fury Abaddon charged the Daemon Prince. He knew he could not best the horror, but he could delay it. Be'lakor deflected the Astartes' blows with casual ease, laughing at the Space Marine's futile attempts as he did so. Soon, the other Battle-Brothers within the Chamber charged the Daemon as well. It laughed and summoned another blade to meet their desperate charge. Dozens of Astartes threw themselves at the monster, but Be'lakor shredded them to bits, dodging bolt shells and bisecting Astartes with supernatural skill the whole time. Screaming with righteous fury as another one of his brother was killed by the daemon, the Redeemer fought on. Abaddon was too slow on his next parry and was sent flying with a lash of Be'lakor's draconian tail.
Abaddon landed in a crumpled heap near his father's body. As he lay next to Horus' corpse, a faint flicker of Light caught his attention. Jolts and sparks of psionic electricity emanated from the Speartip buried in Horus, faint golden energy glowing from it. Eyes locked on the spear-tip Abaddon stumbled to his feet and felt a presence brush up against his mind. His father's voice blasted through Abaddon's head at a volume akin to a thunderclap. "Take up me up and strike down the evil" it commanded. "Purge this unholy daemon, Abbadon the Redeemer."
Slightly concussed and mad with grief, Abaddon did not even question the reality or sanity of what he was about to do. Scrambling over to the Primarch's body, he gripped the speartip, pulling it free from his father's corpse, its psychic energy crystalizing into a longsword of purifying light. A corona of gold rippled out and through Luna. Every psychic on Terra's moon felt it and instinctively knew the source. This golden beacon called out to three in particular: The Angel, Marcus, and Kalib. It called them to the place of their sibling's death.

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Imperium Ascendant
Science FictionWe all know the tale, The Emperor of Mankind creates Twenty Primarchs to conquer the Galaxy. Nine Fall to Chaos's taint and Let the Galaxy Burn. What if this was not the case? What if the Ruinous Powers never got their claws into the Emperors Sons...