Gedou Majutsushi no Hyouitan: Saikyou Kenshi wo Nottottara, Jibun no Karada wo Sagasu Koto ni Natta

A magician who lived for hundreds of years by posessing the bodies of others was strolling through the forest in search of a new vessel when he found a young man on the verge of dying and surrounded by slain monsters. Under the condition that the magician would protect Erena, the boy's companion, he made a deal that they would coexist together in the one body. However, what the magician did not know was that the boy was the swordsman that had been expelled from the hero's party.