The Rise of the Unemployed Wise Man
Karna was a boy that was given the title of "Wiseman" from heaven, is scouted by the hero party that 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 demon, 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.
Shitsugyou Kenja no Nariagari