Shintetsu Nyoi (神鉄如意, literally "The Divine Metal at Will") is a polearm-shaped Treasure Tool possessed by Sydonay. It can change form into any weapon the wielder wishes it to be (even multiplying into thousand of itself). This Treasure Tool cannot be bent, fractured or destroyed by any means, unless it was the wielder's demand. When it first met the revived Snake of the Festival, it shifted shape into the coiling-snake-like altar welcoming his return. It is one of the three instruments for the Grand Order.
In the past, its master once used the name Chi You (蚩尤 Shiyuu ), the name of a war deity who waged war against the Yellow Emperor but was eventually defeated and executed. Ironically, his ultimate adversary is Yu Xuan's contracted Crimson Lord, "Pennant of Offering" Di Hong ("奉の錦旆" 帝鴻 "Hou no Kinpai" Teikou ), which is the other name of Yellow Emperor, himself. Di Hong's divine vessel is also called "Kun Wu" (昆吾 Kongo ), named after Yellow Emperor's sword. Kun Wu, which, according to the legend, it could cut through jade like paper and was used to kill Chi You. It is hinted that Di Hong and Chi You (Sydonay) have fought with each other in ancient times. But in the end, contradictory to the factual legend, in the battle at Shanghai's Outlaw branch, Sydonay personally executed Yu Xuan and drove Di Hong back to the Crimson Realm.
It is used to hunt and kill notable Flame Haze by Sydonay, Karl Berwald , Dorel Kubelík and Yu Xuan. It is also used to lead an army of Denizens and Lords, destroying the major Outlaws throughout Europe and China. It is also used to protect Snake of the Festival during the Second Great War and used to eliminate Centerhill, one of The Four Gods of Earth during one of the battles.
In its master's final battle, he took a huge form resembling a suit of armour and battled Margary Daw in her multi-headed wolf form. It is not used much in the final battle and it disintegrated after its master is killed in that battle.
- Sydonay appears in being, the second opening theme song of the first season of the Shakugan no Shana anime, with Shintetsu Nyoi, even though he isn't seen with it in the entire season.
|Offensive||Blutsauger • Bubble Loot • Chorde • Dance Party • Hystrix • Kiken no Kentō • Kisan no Taiken • Nietono no Shana • Regular Sharp • Traverso • Trigger Happy|
|Defensive||Azure • Bakuyagai • Orgel • Robe • Tarnkappe|
|Utility||Caina • Chef's Paragon • Crystal Altar • Gehinnom • Giralda • Jettatura • Nine Eternal Divine Scales • Nomenclator • Obelisk • Prison of Destruction • Reiji Maigo • Reshuffle • Saishuku no Shadan • Seireiden • Tendōkyū • Tessera|
|Miscellaneous||Atalante • Nachtigall • Trivia|
|Universal||Gordian Knot • Mekest • Shintetsu Nyoi • Tartaros • Trigon|