A brief description of the Dorei Tensei: Sono Dorei, Saikyou no Moto Ouji ni Tsuki manga:
Ars, the prince of the Kingdom of Karitz, who was called the strongest wizard, died at a young age. But on the brink of death, he uses the magic of reincarnation. Reincarnated as Wals Sai, the slave with the best body! And he became an escort of a princess in a small country, became an unmatched slave who uses his fighting skills and magic to crush all enemies!