Zanseiken

 is one of two demon swords of the infinite cosmos now wielded by the dog-hanyō Towa Higurashi. She acquires the sword in the Second Act of Hanyō no Yashahime. It is one of the two swords given by the god of stars,. The other is Kirinmaru's Bakuseiken of dark stellar energies.

History
After Towa plays the Star-Slicer Flute, it turns into the Zanseiken sword and fight against the Demon Spirit Beast of Mount Musubi. As she dodged from the green demon spirit's attack she was surprised that Kyūyōkon can absorb. Finally, she uses her recently developed Twin Azure Dragon Wave, which she channeled through it, to destroy it in one strike. Some nights later, she uses it against some minor rock demons attacking her.

After finally embracing her twin sister, whose memories of her had just returned in full, the Zanseiken is used to break yet another of Zero's large red orbs of demonic energy with one slight swing. Later, Towa had combined the mighty augmenting abilities of the Kyūyōkon Root which it had merged with from the Star-Slicer Flute, with her Twin Azure Dragon Wave to deal Kirinmaru a near fatal blow as it was gradually absorbing his mighty demonic strength. This conjunction of different powers turned the twin dragons a bright green from a light blue-white. With Towa trapped in the Border of the Afterlife, Jaken comes to the conclusion that it is the only thing that can restore Sesshōmaru to full strength and power since he had been hit by its opposing weapon.

Physical description
The Zanseiken has a gold guard in a style similar to that of Kirinmaru's armor with a deep blue gem embedded in it.

Powers & Abilities

 * Supreme Demonic & Stellar Energy Storing: Originating from the infinite cosmos itself, it possess nearly infinite amount of stellar power as well as that of countless yōkai, having being created by the almighty God of Stars himself. However, these powerful abilities and attacks center on pure light and goodness, healing and creation, somewhat similar to the Tenseiga. All of these are equal in strength to its negative counterpart whose effects can be fully neutralized by it.
 * Demonic Energy Absorption & Empowerment: Having the supreme Kyūyōkon Root as part its formation, the Zanseiken can absorb large quantities of demon energy and becomes considerably stronger every second. It is this ability that saved Sesshōmaru's life by reversing the negative effects of its counterpart. However, doing so had seemingly drains its chosen wielder of its very spirit/essence in the process.
 * Soul/Spirit Absorption: When used in conjunction with the barrier of Mount Musubi, the Kyūyōkon Root attached to it can absorb the souls of demons. It is this ability that seemed to have killed its own wielder whom it had chosen.
 * Enhanced Slashing: As it is empowered by Towa's demonic energy, its ability to slice through solid objects is more effective.
 * A stronger and more effective technique used by Towa, from the tip of her energy blade, she can unleash a powerful demonic energy wave in the form of a colossal blue-white twin head dragon, Kirinmaru was the one whom gave it the name. When she fought Kirinmaru for the third time, the dragons turned green due being infused with the supreme demonic powers of the Kyūyōkon Root through which the attack was being channeled.
 * Demonic Barrier: This shield of innate demonic energy was erected to defend Towa from a gigantic rock demon.

Trivia

 * The Zanseiken was teased at the end of the first season finale.
 * The blue stone looks reminiscent to the blue Rainbow Pearl.
 * A special flute known as Hoshikiri no Fue works in conjunction with this sword. When played, the flute transforms into the sword.
 * The Zanseiken is the second yōkai sword shown to transform, the first being Tessaiga.
 * In "Zero's Wish" it was not used when Towa turned into a full demon, but was replaced by her former light blue energy blade from the broken Kikujūmonji replica.
 * The way it has a powerful weapon of equal magical strength and abilities is just like the Sunflower from Tangled, and the Scepter of Light from Elena of Avalor; all three can have their positive and fatal effects undone and neutralized by its opposing weapon possessing all existing negative forces.