Jimi na Kensei Soredemo Saikyou desu

Sansui Shirokuro is a young guy with an old man's name, which ends up landing him in tragic circumstances when God snuffs out his life candle after mistaking him for a geezer whose time has passed. Now reincarnated into a fantasy world, Sansui is one of the Immortals, capable of immense power... as long as he's willing to put centuries of work into...