The Shadow Sword (降魔の剣, Kōma no Ken, literally translated as Sword of Demonic Invocation, VIZ translation, Conjuring Blade) is one of the three Artifacts of Darkness. The sword is crafted from a toxic stone, and a single scratch from this weapon transforms the victim into an E class demon; most likely a cultivated human. Only a special antidote, found in the sword's hilt, can reverse the demonification process. The blade's transformation power has no effect on other demons, as evidenced when Kurama was stabbed by it, and did not turn into one.

Hiei was the one to steal this from the Spirit World vault, but it was later returned when he lost to Yusuke. However, as the blade was coated in Kurama's blood, King Enma learned that the artifacts were stolen and spanked his son for it.

The link for the anime sword: "https://yuyuhakusho.fandom.com/wiki/Shadow_Sword"