Shitsugyou Kenja no Nariagari: Kirawareta Sainou wa Sekai Saikyou deshita

Karna was a boy who was given the title of "Sage" from heaven, and was scouted by the hero party who had great expectations for his skills. However, his skill "necromancer" was creepy and he was immediately exiled. Karna, who got lost on the road without getting a reward, had accidentally invaded the Demon King territory and was targeted by the demons! He faced a demon with mighty power but defeats it with magic that overwhelmed it by far. Meanwhile, Cecilia the devil, who saw the whole scene, offers Karna a large annual salary. And thus Karna decided to become an enemy of humanity as a magician with the Demon King army.

(Source: Shueisha, translated)