Amakuni Yasutsuna (天國 安綱) is the legendary swordsmith who supposedly created the first single-edged longsword (tachi) with curvature along the edge in the Yamato Province around 700 AD.
Although there are almost no modern examples of signed works by Amakuni, legend has it that the double-edged katana, Kogarasu Maru, was forged by this man.
[1][2][3][4] One day, Amakuni and his son, Amakura, were standing in the doorway of their shop, watching the Emperor's warriors return from battle.
Having always looked upon these gestures as a sign of appreciation for his efforts and hard work, Amakuni suddenly noticed that nearly half of the returning warriors were carrying broken swords.
It appeared that the chief reasons for breakage were that the swords had been improperly forged and that the soldiers had struck hard objects, probably armor or other weapons, with them.