Jimi na Kensei Soredemo Saikyou desu
The World's Least Interesting Master Swordsman·地味な剣聖はそれでも最強です
Sansui Shirokuro is a young man mistakenly killed by God, who believed him to be elderly. He is reborn in a fantasy world as one of the Immortals, possessing great potential but requiring centuries of effort to develop his abilities. After five hundred years of sword practice, his routine is disrupted when his master gives him a baby to raise. He encounters a proud noblewoman and her tomboyish bodyguard as he fulfills this task. Though his swordsmanship is unremarkable in style, his extensive experience may prove his skill in the dynamic world of mortals.