Tenka Musou Once More: Isekai no Rou Kenshi, Tensei shite Saikyou Shota to naru

Golden Year 874... After 80 years of dedicated training in the sword, the unrivaled samurai Mifune Siegfried retreated alone into the mountains in his later years, bringing his life to an end in obscurity...or so it seemed, but his second life begins and he once again aims to reach the pinnacle of swordsmanship! A story of a cheating swordsman in another world!

(Source: Shonen Gahosha, translated)