Shitsugyou Kenja no Nariagari: Kirawareta Sainou wa Sekai Saikyoudeshita

A boy named Karna was bestowed the title of "Sage" and recruited by a hero party impressed by his abilities. His skill, however, was as a necromancer, which unsettled the group, leading to his expulsion. Stranded and without compensation, Karna wandered until he unintentionally entered the territory of the Demon King and was attacked. He encountered a powerful demon but defeated it using overwhelming magic. Observing this, Cecilia, a devil, offered him a generous annual salary. Karna accepted and aligned himself with the Demon King's forces, turning against humanity as a magician.