Blood Blade

The, properly known as the Yukari no Tachikiri, is a demonic blade affixed to Setsuna's naginata that first appeared in "The Stolen Seal" in blood form on its wielder's left arm. It was first mentioned by Tōtōsai and confirmed by Jyūbei when questioning about the mark on Setsuna's right arm. It resonates with the Yukari no Tachikiri.

History
When Setsuna was under the control of her demonic blood, her elder twin sister attempted to absorb the excess demon power to try and settle her down. However, a red mark descended on Setsuna's right arm and some purple butterfly wings emerged from her back. Even for when Kin'u reapplied the seal on Setsuna's blood, the red mark didn't disappear.

The next day, Setsuna had her arm checked out by Jyūbei outside the Corpse Dealer's quarters. At the ruins of Izayoi's mansion, Setsuna used the Blood Blade to struck Zero in which she didn't know that she was connect with her mother Rin because of the silver scale curse. During the battle with Kirinmaru, she use the Blood Blade to change blade of the Kanemitsu no Tomoe to red.

Abilities and Techniques
The Blood Blade resides in Setsuna's right arm and can extend be used to slash and infuse energy into blades such as the former Kanemitsu no Tomoe's blade. While the Kanemitsu no Tomoe had merely collided with Kirinmaru's demonic shield, the Blood Blade changes its color to render it ineffective. It is also capable of creating a colossal explosion in both the Underworld and the physical world.
 * Demonic Energy/Shield Penetration: The blade glows bright green and then pierces through increasingly strong demonic barriers, just as Inuyasha's Tessaiga can when it turns blood-red.
 * Dog-Demonic Blood Generation: Just like her half-uncle Inuyasha and cousin Moroha's Blades of Blood ability, it is capable of generating demonic blood and sharpen it into blades capable of slicing through flesh with ease from a distance.

Trivia

 * Much like Bakusaiga coming from Sesshōmaru's body in "Magatsuhi's Evil Will", the Blood Blade came from inside of Setsuna's own body by drawing strength from her ever-developing demonic abilities.
 * This attack is similar to her uncle Inuyasha and her cousin Moroha's Hijin Kessō where they infused their blood with demon energy and used them as blades of blood.