Naisei - Introspection
At the highest point of a human's power, it is said they will attain either one of two things. True peace; The ability to become one with the universe, or True Madness; an insanity so intense it burns their soul a pathway straight to hell. The truth is, neither and both of these things happen. When one gains 100% sychrocity with their soul and inner power, they may learn to unleash their Naisei.
Training for Naisei is an intense process, and most humans simply don't have the time. It could take decades upon decades for them to push their limits to the very edges. It is important to never break the limits. If the limits are broken, Naisei will not be able to activate. When one achieves limitless power, Naisei is impossible. This is why achieving Naisei is so very difficult. One must train as efficiently as they can without truly growing stronger.
Once Naisei is achieved, however, the infintismal growth of the user is inevitable. In order to achieve the state of Naisei, one must learn to summon their Foci without killing themselves. A foci can be anything from a sword or knife, to a pen or coin. A foci in itself is powerful enough to fight against zanpakuto in whatever state the user is capable of. The true power of it, however, arrives when the user embeds the Foci within their still-beating heart.
Once the connection of Focus, Spirit, and Body is made, the user transcends their mortality and enters the Naisei state. This is the final technique a human is capable of using. Once activated, returning to mortal form is impossible, and they shall stay in Naisei form forever, or die and evolve.
The naisei state has no true stereotypical forms (wings, horns, etc). It is completely unique per individual, as it is born from their soul. It's abilities too draw on the user's own soul, and as such, are unique. The only similarity between the transformations is the users' spirit chain. It disappears. Once the soul leaves the body to enter Naisei, the chain becomes pitch black before being retracted into the soul iteslf; the body effectively dead. Once Naisei is unleashed, there is no turning back.