The Demon Blade (妖刀, Yōtō; lit. Devil Blade) is a powerful demon weapon used by the Nakatsukasa clan that was inherited by Masamune Nakatsukasa, while the bulk of the family transformations were inherited by Tsubaki, as though the latter deserved it more for reasons unknown. After defeating Masamune, Tsubaki acquired the Uncanny Sword transformation, thereby obtaining all the abilities that come with it. Tsubaki is wary in employing the Uncanny Sword mode for Black Star, since its demon blade nature could lead to devourment of his own soul, if overused. Only a few limited uses of the form demonstrated just how significant the strain that it places upon his body is, especially on his heart, causing Black Star to grasp his chest and nearly collapse after only moments of use.
The Uncanny Sword takes the form of a katana with a black blade and a bandage-wrapped hilt.
Sword SpiritThis detrimental effect was later revealed to have stemmed from a further presence residing inside Tsubaki, created after inheriting the sword from her brother, which Black Star was unable to effectively respond to. This deer-like entity with prominent antlers and zebra-like stripes, lied deep within its own dimension and when confronted, adopted a very serious attitude, questioning Black Star with "what do you want...kid", before revealing that Black Star's dream had made him take the form of an ogre. During this process, Tsubaki notes that the existence is another wavelength from her family. Due to this unsuccessful attempt at communication, it is only after a substantial period of time, at the conclusion of a trip to the Nakatsukasa household, that a further effort is made. Black Star this time forgoes idle talk and demands that the entity look at his soul. In answer to this, it states that it is borrowing Tsubaki's body, assuming a more humanoid form with the same antlers and stripe patterning. Suddenly, multiple bodies partially rise from the surface below Black Star and attempt to drag him down into the depths, provoking him to ask "What are these guys?". The entity answers "we aim at the strongest souls whose dreams have been defeated", continuing to explain that they are the souls who have been consumed by terror due to the sword. The bodies then adopt ghostly forms, forcibly entering Black Star's mouth, filling him with their despair. However, Black Star stands and chooses to accept their burden, turning away from the path of a demon and instead towards "the path of the sword that revives the dead!" With this, the entity accepts Black Star, allowing him access to it's power.
- It was initially speculated that the presence within the sword belonged to Masamune, Tsubaki's brother but later chapters have suggested otherwise. Firstly, Masamune appears with the darkened faces of the others that have been consumed, seemingly making him just another victim of the sword. Also; the entity refers to itself as "we", suggesting it is composed of all the souls it has collected since it's creation, rather than having just a single identity.
- It is unknown at this point if this entity is similar to the Little Ogre, located within Soul's soul.
- The name "Uncanny Sword" may stem from the concept of the "Uncanny Valley" wherein a person's acceptance of something drops if it looks vaguely but not quite human. Since this is a dangerous mode that changes Black Star's face and makes him more demon-like, it is possible the name derives from the concept.