Blood Blade

The Blood Blade (血刀, chigatana) is a sword wielded by the Inu-hanyō Setsuna, 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. Once completed by Totosai, it is affixed to an end of the naginata and is called the Yukari no Tachikiri. Sesshōmaru then says his youngest daughter's name for it to again act as the medium for sealing Setsuna's feral demonic self.

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.

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, just as the red Tetsusaiga can.
 * Blood Generation: Just like her half-uncle Inuyasha and her cousin Moroha's Blades of Blood attack, 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 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

 * Episode 25 (Hanyō no Yashahime) (fully formed and first use)