Shikōkai (至高解; Supreme Release): This is the Third and true final upgraded form of a Zanpakutō. To achieve Shikōkai , one must be able to materialize and subjugate their Zanpakutō spirit in Bankai within their Inner world; However, It isn't a easy as that. One needs to fight their Zanpakutō to the point of where they feel the two souls "LITERALLY" become one in a resonating furry - Allowing powers that are far beyond Bankai. A Materialization of a Zanpakutō in Bankai within the Inner world means that one has been able to Achieve and actually mastered the Bankai transformation
The power and forms of Shikai, Bankai, and Shikōkai are dependent on the Zanpakutō. They also vary according to the wielder's strength and training. But in general, one's power can grow 15 to 30 times greater. Only the strongest Shinigami can use it. It is also the new ultimate technique of a Zanpakutō. Even the Four noble families, whose members are always exceptionally talented, have a member capable of Shikōkai only every ten generations. Those who achieve Shikōkai always have remarkable roles in the history of Soul Society.
Shikōkai Name: Saishū reien (最終冷炎; Final Frigid Flame)