The Bankai (Final Release) is the second and final upgraded form of a zanpakutō. To achieve it, one must be able to materialize their zanpakutō's spirit in the real world and subjugate it, giving the Shinigami full reign over the zanpakutō's power.
According to Yoruichi Shihōin, the power of Bankai typically increases a Shinigami's power by a factor five to ten. Because such an act usually takes 10 years of combat experience to achieve, it is rare amongst Shinigami and almost exclusively limited to the thirteen Shinigami Captains in the Gotei 13.Byakuya Kuchiki claims that among the four noble houses of Soul Society, only one person in several generations will be born with the spiritual power necessary to achieve it. Even with this, several years of special training are needed to achieve Bankai, plus the experience needed to master it. 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.
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 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.
The power that comes with Bankai is essentially the same as the Shikai's ability/theme but amplifies it to many different levels of power. Some simply increase the original ability, such as Byakuya's blade count multiplied, or Sajin Komamura, who instead of only getting pieces of the giant, summons the giant itself. While other Bankai follow the theme of the sword, such as Renji's remains a whip while containing baboon and cobra features, or Mayuri summons a monster with the same face (Jizō) on his trident, but kept the theme of poison that came from the monsters breath