Katainaka no Ossan, Kensei ni Naru ~Tada no Inaka no Kenjutsu Shihan Datta noni, Taisei Shita Deshi-tachi ga Ore wo Hottekurenai Ken~

Middle-aged Beryl Gardinant has only ever accomplished one thing: running a humble sword-fighting dojo in the quiet countryside. But when he's paid a visit by his former pupil, Allusia, he discovers that she has become an elite commander — and she's hardly the only ex-student who’s a renowned warrior! Now forced by Allusia to become a special instructor to the strongest order of knights in the kingdom, can Beryl reach his full potential even though he's an old man?