Kōkai

Kōkai (高解, Supreme Release): Kōkai is the level above Bankai; though it is not really a third release; merely a level.

Overview
It is not necessarily obtainable for everyone, as very few have been capable of achieving it. What should be noted that it is rare in that the Zanpakutō must possess the special ability required for the form, and the owner of the blade is required to have an absolute love for their Zanpakutō; and lastly, the user's lineage must be deeply rooted in that of royal blood. That being said, when activating Kōkai, the user becomes one with their Zanpakutō, gaining an enormous power boost due to the fact that user has become "whole"; eerily similar to the "Final" techniques of certain Zanpakutō. What should be noted that this release is not a third release; merely a "level" above Bankai; and as such, it is classified as a special technique similar to the "Final" techniques—Reya Toujou is the one who calls it Kōkai. It is said to be a special Reijutsu technique whose name was long-forgotten in the annuals of time; though information has recently told the world that the name of the technique is "Saigo no Reijutsukai" (最後の霊術解, The Final Spirit Technique Release).

Powers
Like with Bankai, when using this form, the Zanpakutō receives a new name based on its upgraded abilities. The power that comes with Kōkai tends to be similar to the Zanpakutō's ability/theme, but amplifies it to much greater levels of power and effect; however, in some cases, the ability is changed due to the user's own soul and personality- for example, a ice-user would gain fire manipulation if they were hot-headed.

The power increase varies from user to user; an age-old warrior could only gain a slight increase due to having mastered everything, while a prodigy with incredible potential could be as powerful as that warrior.