Shinen no Majutsushi: Hangyakusha to shite Yuuhei Sareta Ore wa Fushi no Karada to Saikyou no Chikara o Te ni Ire Boukensha to shite Nariagaru

Wim Abel, the court magician of the Cyrus Empire, is falsely accused of treason by the imperial high command, who fear his mighty power, and he is imprisoned in a dungeon with a strong ward that is considered impossible to break. To pass the time, Wim tries to break out of the dungeon with his overwhelming power, and in the process, he obtains the power of immortality. One year after his imprisonment, Wim finds a slight fissure in the ward and succeeds in escaping to the surface. He gives up on the empire that imprisoned him, and sets out for the Kingdom of Leoria, which is known for its "perfect meritocracy."

(Source: Bunkasha, translated)