Zenkai

Zenkai (禅解, lit. Zen Resolution) is a sword release made as an alternative to Sinsaeng Jeongsu.

History
It is said that only the second generation of Yonkō attained Zenkai during a crisis involving Myoken that would have destroyed all worlds. However, the original Zenkai users did not use even half the power that the technique could have provided, and since then, no one has achieved the level of Zenkai and used it to its full potential. It was later discovered that Zenkai was an alternative level that stemmed from the Sōzōshin Sinsaeng Jeongsu after a non-Sōzōshin attained Zenkai.

Overview
At one point in Shinigami history, Bankai gained a second level allowing full utilization of internal reiryoku. The next logical stage would be to gain a power great enough to the point where nearly no one could detect the user's reiatsu. This power would be all around everything, but it would have zero influence on the environment or anyone.

Unlike Bankai or Shukai, Zenkai focuses on one aspect: the complete mastery of the Final technique. Each Zanpakutō has a Final-class technique that amplifies their power to the absolute limit, discharging it in various ways be it a wave or a manifested form.

The way to obtain a Zenkai is simple when explained, difficult when executed. When someone unlocks the Final technique of their Zanpakutō, they can master it further from training in their Inner World through an uninterrupted Jinzen for 3 decades, 7 years, 10 months, and 12 days, accumulating to 37 years, 312 days. Such time is hard to attain through normal means, so training in the Dangai Precipice World is the only known method to "quickly" obtain a Zenkai. In addition, the person looking to attain Zenkai must maintain their reiatsu at a constant level while at the same time compensating for not consuming anything for almost 4 decades. While this may seem to be an effort for something small, the power of the Zenkai user after going through the training can reach up to 500 to 1,000 times greater than the user's full-power Bankai.

Positives
Zenkai eliminates the strain presented in Shukai, and instead of drawing valleys of power from the user for maintenance, it recycles spent reiryoku to make a constant flow of power. However, it can only keep this up for a short period of time, ranging anywhere from 25 seconds to 25 minutes. Since this technique was not designed for long-term battle, the limits set are quite reasonable.

Limits
Because of the time needed to unlock Zenkai, the user cannot immediately use their new-found power for at least half a year. After activating it for the first time after the training, the user must also spend 5 years to finalize their power to a usable form. Unless this is followed, their Zenkai would dissipate within 10 seconds of activation.

Even after the 5 years are spent to finalize the power of a Zenkai, it is never recommended to use Zenkai as a Wild Card in a battle. While the power that the user emits has no effect on the environment, when the power is focused for a "Final" attack, the density of the air can equate 8,000 m/s², the same gravitational force that is in Asadal, if the user cannot manipulate their power correctly.