Dorei Tensei: Sono Dorei, Saikyou no Moto Ouji ni Tsuki

Ars, the prince of the Kingdom of Karitz, who was called the strongest wizard, died at a young age. But on the verge of death, he uses reincarnation magic. Reincarnated as Wals Sai, a slave with the best body! Wals, who became the escort of a princess in a small country, became a matchless slave who used her best body and magic to crush all enemies!

(Source: Kodansha, translated)