The Bankai (卍解, Final Release) is the second and final upgraded form of a Zanpakutō. To achieve Bankai, one must be able to materialize and subjugate their Zanpakutō spirit. Materialization means the opposite of getting dragged into the Zanpakutō's inner world. Instead the wielder needs to summon the Zanpakutō's spirit into the physical world. It usually takes 10 years or more to achieve, plus the experience needed to master it.
The power and forms of Shikai and Bankai are dependent on the Zanpakutō. They also vary according to the wielder's strength and training. But in general, one's power can grow 5 to 10 times greater. Only the strongest Shinigami can use it. It is also the ultimate technique of a Zanpakutō. Even the Four noble families, whose members are always exceptionally talented, have a member capable of Bankai only every few generations. Those who achieve Bankai always have remarkable roles in the history of Soul Society.
Phazon Pete and Sannse are the only known individuals that have mastered Bankai through a far shorter, yet much more dangerous method. Using a special device created by Sannse, which forcibly materializes a Zanpakutō spirit (thus bypassing a large portion of the training), they were able to attain their Bankai within three days instead of the normal hundreds of years. After successfully testing the device and achieving Bankai, Sannse stated that training by using this method for more than three consecutive days could prove to be fatal, which is why Phazon Pete was also forced to achieve his own Bankai within three days.
As A Varial Definition, Bankai Was Also A Weapon That Also Transformed Phazon Pete Into His Mother, His Father, A Tank, A Stealth Bomber, And A Submarine. The Ritual Transformation Is Actually Called "Banrai", Banrai Is A Ritual Release, Which Turned Phazon Pete Into A Unstoppable Force. Phazon Pete Actually Did Actually Achieve Banrai And Bankai At Central 500, Which The Soul Gods Restricted Him But Ignored Them Completely, At The Core, Phazon Sensed This Power And The Core Actually Came Down Once It Collapsed And After The End, He Mastered Banrai, Which Was A Combined Force All In One. And In Conclusion, The Banrai Was Destroyed In The Infinitian Storm Which Struck Phazon Pete With Millions Of Other Small Forces That He Couldn't Avoid, After Being Struck, He Got Infections Through The Hand And Fingers. And The Expanded And A Sperm Was Inside The Body Of Phazon Pete. Which Will Also Lead Phazon To Combine And Become His Actuall Parents And His Dusk Form Will Be Born.