If It’s For My Daughter, I’d Even Defeat A Demon Lord (Uchi no musume) English Version Light Novel Vol 5

Everything seems to be finally coming together for Latina and Dale. They live in a place where they feel like they belong, they’re surrounded by people they love, and Dale has finally asked for Latina’s hand in marriage.
Before long, though, the devil girl realizes what being the Eighth Demon Lord means: the other demon lords will stop at nothing to destroy her, and that will put everything she holds dear in peril as well. And to prevent that, Latina decides she must sacrifice herself.
However, as a first-rate hero and Latina’s retainer, Dale won’t let that happen, and he sets out to do what he must: eliminate the seven demon lords.