Please someone explain this to me. I am so confused.

I only read this in the comments,
I think the one that kill the heavenly demon regressed but in the process he died because he can't handle the constellations power so it transfers to the one he was with in the gate...

^as they said, and the person he was with is actually the heavenly demon, so the villain in the future got the power that was meant to kill him lol