Zanseiken

"The Zanseiken is a blade that pairs with Bakuseiken wielded by Kirinmaru. In ancient times, Amatsumikaboshi, the god of stars, gifted two swords. I have heard that the Bakuseiken and the Zanseiken were those two swords."

- Rion

was one of two demon swords of the infinite cosmos now wielded by the dog-hanyō Towa Higurashi. She acquired the sword in the Second Act of Hanyō no Yashahime. It was one of the two swords given by the almighty God of Stars, Amatsumikaboshi. The other is Kirinmaru's Bakuseiken of dark demonic-stellar energies.

History
According to Kirinmaru's undead daughter Rion, in ancient times, Amatsumikaboshi, the almighty God of Stars, had gifted two stellar/demonic swords- Zanseiken and Bakuseiken from the infinite cosmos which were sent to.

Following his daughter's demise, Kirinmaru had sought out a suitable body for Rion's spirit to inhabit. To ensure this, he cast a curse onto the sword where the new user's kon would gradually be drained dry in order for Rion's soul to be transplanted into the empty shell.

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: Kirinmaru's Demon Fireball.

However, utilizing its demonic energy-absorbing/storing abilities comes at the ultimate price of losing one's very kon and falling into a comatose state for the essence of one who had returned to life to inhabit their mindless and soulless body, something that Rion had flatly refused to do to the person she was so fond of, due a curse Kirinmaru had placed on it centuries ago after Rion was murdered by a vengeful half-demon Sakasa. Disregarding her grandmother's warnings, Towa vowed to fully master its infinitely powerful abilities in spite of the curse. She admitted that using it a little bit was okay for her.

When facing the Grim Comet's destructive power, it was used to absorb all of the infinite demonic and stellar energies into it, which naturally continued to drain away Towa's kon further due to the fatal curse Kirinmaru had placed on it for the sake of his undead daughter. Having used it many times as the rouge Beast King's right arm, Osamu Kirin takes the sword of the God of Stars and declares that he will use its infinite abilities to put an end to his other half. Earlier, it had further strengthened Towa's signature attack into the considerably more powerful Shooting Star Azure Dragon Wave when combined with the infinite stellar energies from the Grim Comet itself.

Physical description
The Zanseiken has a gold guard in a style similar to that of Kirinmaru's armor with the deep blue gem from the infinitely powerful, mystical Kyūyōkon Root embedded in its center.

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.
 * Supreme Demonic/Stellar Aura: It emits a pale blue aura whenever it it used, though this changes to bright green whenever combined with the evolved Twin Azure Dragon Wave attack of its chosen wielder.
 * 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. This had further strengthened and evolved Towa's attack she inherited from her dog-demon father into the larger, more powerful Shooting Star Azure Dragon Wave.
 * Enhanced Cutting/Slashing: As it is empowered by Towa's demonic energy, its ability to slice through solid objects is more effective.

Additional Abilities
Curse Inducement: Even though it was made from an infinitely powerful god, it is not susceptible to curses of any demonic nature as one was placed by Kirinmaru himself some time after he had taken possession of its negative counterpart.
 * Mind/Consciousness Absorption: Combined with the immense demonic energy of the strong barrier around Mount Musubi, the Kyūyōkon Root attached to it can absorb the energies and very essence of yōkai. It is this ability that seemed to have absorbed the very kon of their host, rendering them comatose. This is actually the negative effects of a curse that Kirinmaru himself had placed on it to ensure the everlasting survival of his beloved undead daughter's very spirit/essence within the physical self of the one who had "stolen" the sword from her, and not an ability/technique of the star sword itself, since Towa absorb the demonic energy of Kirinmaru's haku of Demon Fireball to save her father Sesshōmaru.

Channeled Attacks and Techniques

 * This stronger and more effective attack is actually the evolved from Towa's signature attack. From the tip of her demonic 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.
 * Creation of Demonic Energy Barrier: This pale blue shield of demonic energy was erected to defend Towa from a gigantic rock demon. Like Tessaiga's red demonic shield-shattering technique, it is unnamed.
 * When in conjunction with the infinite cosmic energies of the Grim Comet itself, it glows with a sparkly glow.

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 the Star-Slicer Flute 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 "A Place (Not) for Towa" and "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.
 * This weapon is similar to the Dragon-Scaled Tessaiga as both blades are capable of absorbing yōki and would hurt the wielders if too much yōki is absorbed. However, this negative effect from this sword is the result of a curse placed on it by Kirinmaru himself.