I like what you're thinking
Interesting but why would his demon try to block the dark triad from their goal ? Would he be impossible for him to leave in this world with a human body after they open the afterworld ?
then if that's so, what about the others that also got a devil? what I really do want to know is what's with Asta and where is he from to basically have no powers in a world where everyone has..
He's probably another rouge demon like the a
ATD. ATD doesnt even seem like he's evil at all and he's even hostile to other devils. Seeing how he was infuriated by asta losing to dante. We never seen ATD act like that about any of asta's other enemies except against dante, the host for the strongest devil. So we can assume nacht is like that as well.
I think the original nacht is gone. Original meaning the human soul. I think it's a devil controlling the body now. Kinda like how asta gave his arm to the antimagic, nacht gave his soul to his demon. And now that body can hold multiple devils.