The Nanashi is the result of an Ubume and a Yamaarshi being merged into one monster. The plot is overseen by a Black Puppet, enlisting the aid of the two Makamous' parents to bring the creatures to Asama Mountain. From there, the two Makamou battled each other until the Ubume drove itself into the Yamaarashi's neck, causing it to collapse to the ground with neither showing sign of movement. From there, the Black Puppet forms a cocoon over the two Makamou that merges them into one with the chrystalis.
Emerging from the cocoon as Ibuki and Todoroki arrived, the two Oni having being hunting it components, the newly-born Nanashi overpowers them while proving to be immune to their Onigeki attacks. However, Hibiki arrives to his comrades' aid, cutting the monster's tail off before the three Oni use their signature attacks from all sides to destroy the Makamou.
- The kanji that make up Nanashi means "Nameless", which plays on it being a one of a kind monster the Oni have not encountered before.