Sealed Armament: Izayoi

The Sealed Weapon Izayoi is the weapon of choice for Tsubaki Yayoi. The Izayoi is kept under a special lock that can only be opened from the blood of a person from the Yayoi Family. Tsubaki needed the Izayoi to combat both Jin Kisaragi and Noel Vermillion, since they both possess Nox Nyctores-class weapons. The Izayoi takes the shape of a short sword resembling a quill tip and a book-like shield; however, it can change its shape for some of Tsubaki's attacks. It also has a different true form.

Created long ago to fight the Observers, it was chosen as the prototype for the Nox Nyctores that were produced later. While the Izayoi increases the user's power exponentially, it has a negative side-effect: as it allows its owner to control light, it steals the light from the user's eyes. Due to this, using the weapon for an extended period of time will cause the user to go blind, or in the worst case, it will cost the user's life. The fact that it causes people to go blind is told during the course of Continuum Shift in both Story and Arcade Mode. As a matter of fact, Tsubaki is told numerous times to get rid of it. It is also revealed in Tager's Help Me, Professor Kokonoe! that along with its wielder's light, it draws in all waves within the electromagnetic spectrum, including the radio waves received by Tager's radio, allowing the weapon to interfere with radio signals. Any wielder of the Izayoi are capable of deploying the Zanki Barrier; an extremely powerful Ars Magus.

The initial state is called the pupal or chrysalis state, and its role is that of a shield that prevents interventions or observations. On the other hand, its true released form is called Zero-Type Izayoi (零織兵器・十六夜), and it is in this form that it is able to demonstrate its primary power as an Anti-Observer Weapon (対観測者兵器), its role being that of a pike whose power is the Immortal Breaker, the ability to slay "those that are outside of logic".

Hazama's primary purpose in manipulating Tsubaki to fight her comrades was to gather its power until it overwhelmed her, bringing it under his control, and then creating a space through it that neither Takamagahara nor the Amaterasu Unit were able to observe.

In Phase Shift 4, it was revealed that the Izayoi was wielded by Ayame Yayoi, Tsubaki's ancestor during the Dark War.

In Iron Tager's Continuum Shift story mode and Ragna's Chronophantasma Arcade mode, it is explained that the Sealed Weapon Izayoi is actually a prototype to the Murakumo Unit, explaining that the latter's experience with the Murakumo Units was why Tsubaki stood no chance against him. The first known Murakumo Unit was produced before the Dark War and along with Prime Field Device Type Zero and Azure Grimoire, was used to create the first Kusanagi.

Trivia

 * In Tsubaki's bad ending, she dies due to the overuse of the Izayoi, as she mentioned that she already knew that it will take her life. However, this is not canon. In Noel's arcade mode, when she faces Hazama, he comments on how Tsubaki's strength is "fading", implying that eventually the Izayoi may kill it's wielder if used too much.
 * The name Izayoi means "sixteen-day-old moon" in Japanese. A sixteen-day-old moon has just passed its fullest phase, so it is at this point that its brightness begins to decline. This is another clear reference to Izayoi's blindness-inducing nature.
 * Hakumen was the one that implored research of the Izayoi.
 * Zero-Type Izayoi's portable form is similar to Lux Sanctus, although instead of being a big sword, it is a big shield with a pike encased in it.
 * There is a visible "00" mark on Zero-Type Izayoi's portable and armor form, representing that it has been chosen as the Murakumo Unit's prototype.