Speculation:
Saint Edelweiss became a Demon Lord after Takeru got executed because [fuck you for killing my man]. The pointy ears are demonification.
She's going to offer to stuff him back in his original body, which is a plus for him, because then the original Takeru in that body will be free.
However, the price will be something he can't countenance, like becoming a demon, or killing humans, or whatever.
This will eventually provoke him into a fight with her.
There also has to be elements that bring in the adventurers? Heroes? whatever from the normal world (otherwise what's the fucking point of building them up so much?)
Classic tropes.