They are making a baby there, to be accurate Hyunjae's Future self from AU did it.
future/ older hyunjae (from previous timeline) couldn’t coexist with current (new timeline) hyunjae as current hyunjae completely blocked him out or rather left him behind. so he decided to embed himself in the mana stone as it would allow for it to activate and for a being to be born rather than for him to just disappear forever.
they didn’t actually go back in time in terms of completely erasing the previous timeline but rather this timeline rewrites/ is placed over the previous timeline. so their previous and current timeline selves must coexist and most don’t even notice a difference but hyunjae did and rejected previous hj without even being aware of it
Ooh okay makes sense thank you so much, I remembered only a few parts that really helped me
Guys I forgot what happened, please can someone explain me why the mana stone has some of his mana