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.
Ichigo Kurosaki and Kisuke Urahara are the only known individuals that have mastered Bankai through a far shorter, yet much more dangerous method. Using a special device created by Urahara, 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, Urahara stated that training by using this method for more than three consecutive days could prove to be fatal, which is why Ichigo was also forced to achieve his own Bankai within three days.
Besides Ichigo Kurosaki, the only non-captain Shinigami known to have achieved Bankai are Renji Abarai (Lieutenant), who finalized his training at about the same time as Ichigo (although he did not use Urahara's device, but rather perfected it while Ichigo was training), and Ikkaku Madarame (3rd Seat), who did so through his own years of training.
Bankai Forms: After a Shinigami is able to materialize and subjugate their Zanpakutō's spirit, the Zanpakutō can assume its final and most powerful form. Unlike the Shikai, there is no command phrase to activate Bankai, though the user will usually say "Bankai" beforehand. A Zanpakutō's Bankai is often a manifestation of the spirit itself, usually creating a massive creature or a very powerful effect to aid the user in battle (Ichigo's Bankai is an exception, becoming more compact instead). The Bankai itself will receive a different name in addition to the ordinary Zanpakutō (for example Ichigo's Zangetsu becomes Tensa Zangetsu, Byakuya's Senbonzakura becomes Senbonzakura Kageyoshi). Certain Shinigami also change with their Zanpakutō, gaining clothing similar to what their Zanpakutō's spirit form looks like. Most Zanpakutō will also gain additional special abilities, or a more enhanced version of the usual one. Like with the Shikai, most of these abilities have an associated name. Maintaining the Bankai is taxing for the wielder, and it is difficult to maintain it for long periods of time for most Shinigami, with the exception of Ichigo, as it becomes more compact, thus allowing him to hold it for long periods of time.
Special Abilities: The power that comes with Bankai tends to be similar to the Shikai's ability/theme, but amplifies it to much greater levels of power and effect. Some simply increase the original ability, such as Byakuya Kuchiki's blade count multiplies greatly, Ichigo Kurosaki's Getsuga Tenshō becoming faster, far more powerful, and its color become black, or Sajin Komamura's, who instead of only generating pieces of the giant, summons the whole giant itself. While other Bankai follow the theme of the sword, such as Renji's remains whip-like while containing baboon and cobra features, or Mayuri summons a monster with the same face (Jizō) on his trident-esque sword, but kept the theme of poison that came from the monster's breath.
Weakness: A high enough level of spiritual energy can be used to disrupt the rhythm of a Bankai. The biggest flaw of Bankai lies in the overwhelming power it releases. Its power and form are often far beyond that of a normal weapon. This is the reason that in order for one to become capable of using it completely and effectively, one needs to train at least 10 years with Bankai. Any Bankai before that period of time has been reached should not be used in battle.