It's an unusual style in that Inosuke uses two Nichirin Swords ... Moon Breathing is a style practiced exclusively by the Upper One demon Kokushibo, who was once a Demon Slayer before discarding ...

With the release of Hashira Training Arc, the Demon Slayer ... Tanjiro's Nichirin sword. This allowed Muichiro to return to the fight and defeat the Upper Moon Five demon with Mist Breathing ...