Oh, it's slightly different from the novel.

Spoilers pls

Mostly the blonde died before the party ever arrived and MC is not actually trapped in the mirror and just pretending which is something Terry already knows, because he knows her well, though Terry was still on edge during this even in the novel the manhwa made it more dramatic.

I think it's a nice cliffhanger for the people who don't read the novel, since the suspense makes you wanna anticipate what's going to happen next. Although I agree that there are slight changes from the novel.