Jimi na Kensei wa Sore Demo Saikyou desu (Novel)

Sansui Shirokuro is a young man with an elderly-sounding name, leading to unfortunate events when God extinguishes his life candle, mistaking him for an old man whose time has come. Reincarnated into a fantasy world, Sansui becomes one of the Immortals, wielding immense power if he dedicates centuries to mastering it. After five hundred years of relentless sword practice, his routine existence changes dramatically with the arrival of a baby. His master tasks him with raising the child, leading him to encounter a proud young noblewoman and her tomboyish bodyguard. Though his fighting style is simple and unassuming, the fast-paced mortal world will soon discover how formidable even the most unlikely swordsman can become with half a millennium of practice!