Nenekirimaru (祢々切丸; Literally meaning "Ancestral Shrine Round Cut Off") is an exorcism blade which only harms yōkai. It was originally created by the 13th head of the Keikain family, Hidemoto Keikain, and is given to Yōhime by Koremitsu Keikain as a precautionary measure against the yōkai running amok in the capital. Nurarihyon ends up with the weapon when she is kidnapped, and he uses it to defeat Hagoromo Gitsune. The blade is eventually handed down to Rikuo. Though the weapon is considered incredibly important by both Onmyōji and those of the Nura House, Rikuo and his attendants occasionally used it to play blindfolded watermelon-chopping games when he was younger. At one point, the sword is stolen by Oitekebori and during Rikuo's stay in Tōno, the Namahage keep possession of it. It is later shattered to pieces following the Nue's rebirth.
Rikuo sought Keikain Akifusa's help in reforging the Nenekirimaru, it is seen that Keikain Ryuuji informed Rikuo on Akifusa's behalf that the Nenekirimaru is ready.
Creation
The sword name was originally called Nuekirimaru (鵺切丸, Nuekirimaru). Hidemoto Keikain called the sword like that to telling his ancestors in the future that he is leaving the best sword that he created, but it has been turn out as fate. Ryūji explained that it's no exaggeration for history to say that the Keikain s intent of "The Nue's defeat" has been engraved in the sword itself. It is a sword infused with a spirit of an Onmyōji. Created in a way that it increases its powers when it cut a spirit or yōkai.