Blood Blade

The Blood Blade (血刀, chigatana) is a sword wielded by the Inu-hanyō Setsuna, that first appeared in The Stolen Seal. It was first mentioned by Tōtōsai and confirmed by Jyūbei when questioning about the mark on Setsuna's right arm.

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 calm 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.

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 Kanemitsu no Tomoe's blade. While the Kanemitsu no Tomoe hit Kirinmaru's barrier, the Blood Blade changes its color to destroy the barrier.
 * Penetration: The blade glows bright green and then pierces through demonic barriers.

Trivia

 * Much like Bakusaiga coming from Sesshōmaru's body in Magatsuhi's Evil Will, the Blood Blade came from inside of Setsuna's body.
 * 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.

Season One

 * Episode 22 (Hanyō no Yashahime) (first appearance; as a blood mark)
 * Episode 23 (Hanyō no Yashahime) (first indirect usage)
 * Episode 24 (Hanyō no Yashahime)

Season Two: The Second Act

 * TBA