Sadahide (定秀) is recorded as a monk (so) residing at Mount Hiko in Bungo Province, active from the end of the period into the early period. He belongs to the Kyushu Classical group of swordsmiths, and tradition inconsistently describes his relationship to Yukihira -- some sources calling him Yukihira's teacher, others his disciple. No dated examples are known among extant works, and the acknowledges that his signature style is not uniform, leaving room for further study as to which form should be regarded as typical. The long signature "Bungo no So Sadahide " appears on several surviving , though its scale and character vary between examples.
Sadahide's workmanship exhibits the distinctive qualities of the Kyushu Classical tradition. The forging is a well-refined , frequently compared to Yukihira's, with a tendency toward flowing or standing grain and a somewhat whitish cast to the steel. Fine adheres well, enter, and a suggestion of appears. The is a narrow carrying a slight tendency, mixed with elements. The is characteristically described as urumu -- moist or softened -- with thick , and features such as uchi-noke, , , and run throughout. The blade forms typically preserve pronounced with clear and a , presenting an archaic and elegant shape. Carvings, when present, include run through in and, in one notable example, the character katsu with cherry blossoms -- a motif also associated with Yukihira.
The praises Sadahide's -based workmanship for conveying "the gentle, restrained charm typical of Kyushu works." Signed examples are acknowledged as extremely few, lending each exceptional documentary value alongside its characteristic . One blade is recorded as having been presented by Emperor Go-Mizunoo to Mori Hidechika, attesting to the esteem in which these works were held. The combination of graceful form, softened , and moderately -laden tempering places Sadahide firmly within the understated aesthetic of early Kyushu sword production.