Henkyougurashi no Maou, Tensei shite Saikyou no Majutsushi ni Naru

A powerful mage known as "The Lone Demon King" lived at the frontier, regarded by villagers as a guardian deity despite being an ageless entity. He was eliminated by a magical organization that viewed him as a threat. Two centuries later, he is reborn as the second son of an aristocratic family and gains the most potent magical abilities. He intends to live as a human this time, though he is not driven by a desire for vengeance. Having once served as a regional guardian, he now lacks basic human knowledge and struggles with normalcy. As an aristocrat, he is cherished by his family and peers, and is recognized as a top-tier magician within the guild. Unbeknownst to others, he has become an "Immortal Mage."