The Uncanny Sword (妖刀, Yōtō) is a demon weapon form transformation of the Nakatsukasa Clan.Masamune Nakatsukasa had inherited this transformation from his ancestors. After his death, the Uncanny Sword is then absorbed by his younger sister. 
The Uncanny Sword takes the form of a katana with a black blade and a bandage-wrapped hilt.
One of the Uncanny Sword's known abilities is "Soul Possession", in which exploits the fears in people's hearts to possess the body and soul of the person who carries it. In the end, it absorbs the person's soul entirely. The appearance of the individual changes; they gain black lines around their face. Those possessed also take on red eyes. The Uncanny Sword also can control the shadows of its host.