There was a mage called "The Lone Demon King" at the frontier. He was just a creature that never grew old, but he was loved by the villagers as something of a guardian deity. However, he was destroyed by a magic organization that sees his existence as dangerous. 200 years later, he is reincarnated as the second son of an aristocrat and obtains the strongest magic power.

"Let's pretend to be a human this time..."

However, being a human lover, he didn't particularly think of revenge and has decided to pretend to be human in this life. In his past life, he was the guardian deity of a region, and in this life, he's a local aristocrat, but he does not know common sense. He doesn't know how to take it easy. As the second son of an aristocrat, he is loved by his family and friends, and while he is regarded as a master of magic in the magic guild, he becomes an "Immortal Mage" behind the scenes.

(Source: Shousetsuka ni Narou, translated)