I remember playing an eroge a while back but I only remember the basic premise.

Basically, the protagonist is a guy who is part of a group of magical girls at school or something like that and he assists them. However, he doesn't have any powers if I recall correctly and so the team treats him bad. When they beat up a monster, it leaves behind a book or grimoire and the protagonist takes it home and it turns out it actually is host to some dark Lord or something like that. It offers him powers and he accepts and so he uses it to get revenge on the team.

Sorry for the vagueness its been a while since I played it. Thanks!