Lesson from Sword Emperor (剣帝の教え) is the 123rd episode of the Katekyō Hitman Reborn! anime series.
Yamamoto and Genkishi continue to battle, until Yamamoto's sword appears cracked. He then remembers in the video Squalo sent him of Genkishi, that he mentioned mist illusions have gotten to the point it can trick everyone in the surrounding area and even cameras. Meanwhile, Gingerbread and Iris continue searching for Spanner. Yamamoto realizes that Genkishi created the illusion of his sword being cracked and continues their battle.