Dorei Tensei: Sono Dorei Saikyou no Moto Ouji ni Tsuki

Ars, the prince of the Kingdom of Karitz known as the strongest wizard in the world, died at a young age. But on the verge of death, he uses reincarnation magic to reincarnate as Wals Sai, a slave with the strongest body! Wals becomes the escort of a princess from a small kingdom, a matchless slave