I’m Not a RegressorNot a Regressor

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we see that mc doesn't have the full black sky and there is another actually heavenly demon that also has a black sky and he is hiding and needs to kill the heaven-defying star(returnee) and absorb his power to complete his black sky

Ok theory time I think that lee shin hyak guy is the actual heavenly demon and sealed his memories like split personality thing or another body thing and hiding among masses and proped up MC as a heavenly demon in his previous life because as we can see
in the previous timeline
1)he is abnormally stupid for an awakened
2)he also forgot that he killed mc (Left to die)
3)also subconsciously tries builds a relationship with haeun when she comes to the gate to find MC (who is left to die) she is hot but she is also rare 7 stars awakened of dragon constellation with that weird dragon eyes (cursed at the beginning of the novel) that help in controlling dragon heart
and doesn't connect the dots that it's the same guy (my because of MC's lies, he is dumb after all)

current timeline
4)We know that memories get raised when we use active black sky powers, that is how shin hyak must have survived the dungeon
5) After regression he is seen to have a creepy smile on his face despite his plan failing
6) Also when he enters the demon kingdom he hears a male voice noticing his arrival
I think in previous time line, at the end when he defeated the Ojin heavenly demon absorbed his power (because black sky is cheat it can hide your absorption) he was sill not power full enough to absorb vega so he wanted to go back in time with all this knowledge and die erasing shin hyak the sub body and returning to main body with all the knowledge with cutting off his trail there. He has all the knowledge while MC only receives snippets in shin hyak's pov, he is fattening MC like a sheep through those memories. Other wise the actual Hevenly demon would have noticed MC as the heaven defying star due his growth rate and would have took action.

serpent queen