From Kodansha:
Allen, a powerful sorcerer feared as the “Demon Lord” by all in town, wants nothing more than to enjoy his solitary existence deep in the forest. That is until the day he discovers a young woman lying unconscious in his neck of the woods and saves her from a band of knights. He soon learns the girl, Charlotte, once betrothed to the prince of the neighboring kingdom, was framed by her fiancé for vicious crimes and forced to flee her home. Moved by her pitiful plight, Allen stakes his own life on a promise. To give Charlotte a crash course in all that is naughty and make her the happiest woman in the world.