The Uncanny Sword (妖刀, Yōtō) is a demon weapon form transformation passed down by members of the 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.
After gaining the transformation, Tsubaki is capable of changing to this form via Tsubaki Mode Change: Uncanny Sword (椿のモード変化「妖刀」, Tsubaki no Mōdo Henkō "Yōtō").
The Uncanny Sword takes the form of a katana with a black blade and a bandage-wrapped hilt. In the anime, the Uncanny Sword's true form is expressed.
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.
In it's true form, the Uncanny Sword gives the Meister a tremendous boost of power. This form is only attained after acceptance of the Will of Nakatsukasa, in which allows the full power and usage of the transformation (and therefore it's related techniques) without ill effect.