So I don't know if I want to make this canon for this story, but as a fun writing exercise to get my creative juices running, I wrote this. I hope you enjoy and please don't consider this canon until I say otherwise :)
Ezekyle Abaddon, more commonly known as Abaddon the Despoiler, stared at the cognitor displaying the results of the ongoing battle. The overwhelming force he had prepared was more than enough outnumber and outgun the defenders of Cadian Sector ten times over. The Imperial defenders should have been able to withstand his forces for several days at best or a few weeks according to his worst expectations.
This expectation had come from the fact pulling the same trick had had done during the 12th Black Crusade, he had managed to isolate the Cadia Sector with Warpstorm, preventing Cadia from receiving the much needed reinforcements and supplies to withstand Chaos.
Instead, the battle had dragged on for months then Abaddon had expected.
All his plans had been thwarted when one Imperial Battleship, no more accurately the man in charge, had appeared out of nowhere just as the battle over Cadia started.
Abaddon growled. The Imperial Admiral that should have died centuries ago following his orders to sabotage the Warp Drive and Gellar Fields of the Hero of Gothic Wars' Battleship. His intelligence network had reported that the man to be missing alongside his crew and battleship for centuries so why did Spire have to miraculous appear when he started 13th Black Crusade?!
"Warmaster, the Blackstone Fortress reports that it cannot function anymore!"
The Warmaster of Chaos Undivided brought up the new information to his cognitor. He should have expected this when Admiral Spire had taken command of the Imperial Navy defending the Cadian Sector.
Well no matter, Abaddon smirked as he prepared to give the order that would make the purpose of the 13th Black Crusade a success. "Have the Blackstone Fortress to collide with Cad-"
"I am very disappointed in you, Abaddon," said a voice that shocked not just Abaddon, but everyone that had once known the speaker.
"What trickery is the Corpse-Emperor trying to pull now?" retorted Abaddon as he quickly recovered. "I would never fall for such a petty-"
"Oh, I completely forgot this isn't the timeline I had come from," continued the impossible figure. "Oh dear, this is confusing for me, but it must be more so for you." The gigantic figure started to smile, reminding Abaddon of his Primarch before Chaos had corrupted. "I am not the Horus Lupercal, traitorous Warmaster of Chaos, that you are familiar with. I am the Loyal Warmaster that stopped the traitorous Roboute Guilliman and died by sacrificing my life-force so my Father, the God-Emperor of Mankind, may survive even in a crippled state."
Abaddon was about to chuckle at the absurdity he was hearing when a part of him treated the newcomer as a serious matter. At first, this doppelganger seemed like the loyal and pure Horus Lupercal before his fall to Chaos. That quickly didn't seem to case Abaddon started noticing physical differences the doppelganger's appearance. This doppelganger possessed scars and slight differences in his appearance that Abaddon didn't remember his gene-sire possessing. If it was just his physical appearance then Abaddon would have quickly dismissed this to be a fake as he had encountered too many demons, xenos and pyskers with the ability to shapeshift.
Something screamed that it was not a doppelganger, or even a clone like Fabius could have created. The very presence that the fake emitting screamed that this was Horus Lupercal!
No, this wasn't Abbadon's Horus Lupercal.
The first thing Abaddon noticed was the eyes. This "Horus Lupercal" had eyes that only someone that had lived for thousands of years or more could possess. Something this his gene-father could never possess as his Horus Lupercal had lived no more three terran centuries.
Yet at the same time possessed everything Abbodan remembered of a Horus before being corrupted by Chaos, and even more.
This was like Abaddon was looking at Horus Lupercal if he hadn't fallen to Chaos and remained loyal to Imperium.
"That's right, my son from another timeline." Horus nodded. "In the place of the pathetic failure of mine from this timeline, I sentence you to death for bringing the end to the enlightenment of the Imperial Truth! May you find nothing more than eternal damnation as Alaya condemns you to never-ending service as a Counter Guardian. A life where you are forced to forever fight against the very Chaos you swore loyalty to!" Horus brought his gigantic Power Maul Worldbreaker, said to be constructed by the Emperor's own hands, at Abaddon. "For Alaya! For the Imperium! For Humanity! The Emperor Protects!"