Tensei Kenja no Yarinaoshi: Ore Dake Tsukaeru Kikaku-gai Mahou de Nidome no Jinsei wo Musou suru

For his entire life, a great sage devoted himself to the mastery of magic with a singular purpose: to vanquish the Demon King. After years of relentless preparation, their long-awaited confrontation finally takes place. However, rather than face defeat, the Demon King unleashes an unexpected reincarnation spell and flees into the distant future. Refusing to let his lifelong mission end in failure, the sage immediately follows his nemesis through time, determined to finish what he started no matter the era. Five hundred years later, the sage awakens in a new body as Ren Strain. As he begins to navigate this unfamiliar world, he quickly uncovers a startling reality: the magical arts that once defined civilization have suffered a catastrophic decline over the centuries. Spells and arcane knowledge that were once considered basic are now entirely lost or vastly diminished. Armed with his unparalleled expertise from a bygone age, Ren finds himself wielding power far beyond anything the modern world has ever seen. With the Demon King still at large in this weakened era, Ren must use his extraordinary abilities to dominate his second life and finally fulfill his ancient vow.