Katainaka no Ossan, Kensei ni Naru: Tada no Inaka no Kenshutsu Shihan datta noni, Taisei Shita Deshi-tachi ga Ore o Houtte Kurenai Ken (Tateyomi Furukaraa-ban)

Middle-aged Beryl Gardenant 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, Allucia, 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 Allucia 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?