AHA! i found the damn theory:
"Abby have long been rumored to being the clone-son of Horus in the fluff.'
HIDE SPOILER!'
When he faces Horus Reborn, theres practically -no- difference between them, save the armor. And Khayon muses that perhaps Horus 2 wasnt the only clone present. I speculate that the old belicose and choleric Abbadon simply did not survive the Horus Heresy. Horus thusly had a clone created from himself as a template to insure a less emotionally unstable heir. Perhaps making him as a failsafe prior to confronting the Emperor, to make sure there was a proper backup for legion leadership incase he got trashed by his father."
Now look at mah boi ADB:
Are you Horus? Are you his clone? His… son?''
He laughed, the sound wet and tinny over the speakers.
'What do you believe, Khayon? Do you think I am?''
I saw no reason to lie. 'Yes.
''This delighted him. I was not sure why.'
'And if I were, brother ― if I were merely Horus remade, recrafted, with a twist in my gene-code here and an alteration there, would it change anything?''
I had to think about that. I looked into his eyes but saw no answers there, only amusement.'
'Perhaps. Perhaps you have always been a genetic twin of your primarch. Or perhaps Ezekyle Abaddon was slain in his pilgrimage across the Eye, and you are one of Fabius' creations in his place. How am I to know?'
...
'So it seems.'''Let me ask you this, Khayon. What does it matter? Clones, sons, fathers… Let the herd whisper whatever truths they choose. Our eyes are set on worthier goals. We look to the future, not the past.''I acceded with a nod, knowing that there was no answer to be
had here. Knowing, ultimately, that he was right. It did not matter.
