Breath of the Sun

The Breath of the Sun (の) is an as-of-yet-unseen style of swordsmanship. It was utilized by the first Demon Slayer to use breaths, Yoriichi Tsugikuni, and serves as the basis for other Breath Styles derived from it as a result of Yoriichi changing the breathing techniques to suit the individuals he taught. The derived styles eventually branched out into the main breaths: Breath of Water, Breath of Flames, Breath of Wind, Breath of Thunder, Breath of Stone and Breath of the Moon. Before Kokushibo died, he mentions that all swordsman who knew of the Breath of the Sun were killed by him and Muzan Kibutsuji, before proceeding to question why the Breath of the Sun still exists due the possibility that the Dance of the Fire God is possibly the Breath of the Sun.

Furthermore Shinjuro Rengoku independently identified the Dance of the Fire God as the Breath of the Sun through the memoirs of his Demon-slaying ancestor. The possibility that the Dance of the Fire God and the Breath of the Sun is the same breath is further suggested by fact that there is an unknown 13th form, which can be learned from the Breath of the Sun as a Dance of the Fire God User.

Trivia

 * The kanji for "sun" has the same pronunciation as the kanji for "fire".