I can't believe I had just found this title. It's the first time in a while since I've last read an isekai where it isn't unbearably cliche. This title just felt sweet, and like come on, the girl is in all the cover art. I just wished there wouldn't be a harem.
Guessing his game version used his demon sword ability to absorb the Saintess' magic crystal and whatever the core of the headmaster's power is, for the sake of saving the world maybe?