Kurikara

Kurikara is a Demon-slaying sword originally held by the Myōō Dharani sect. The founder of the sect, Fakaku, infused the Kurura inside the blade and with it, brought down the Impure King around 140 years ago. However, according to legends, Karura then left the sword, after the feat.

History.
Shiro Fujimoto recieved Kurikara from Tatsuma Suguro after he abruptly intrudud when Tatsuma's wife was giving birth.

Shiro used the blade, with the aid of Mephisto, to seal Kazumi Okumura's demon heart into the Kurikara at Birth, thus fusing it with his blue flames inherited from Satan. This let Kazumi stay a human for fifteen years until the sword could no longer contain that power. It is now under possession of Kazumi, which he can unsheath Kurikara to restore his demonic nature and powers.