Depending upon the Shinigami's ability to communicate with and control their Zanpakuto's spirit, a Zanpakuto can manifest itself in two additional forms and revert to a sealed state. These two forms, known as Shikai and Bankai, are akin to "upgrades" for the Zanpakuto, giving it abilities far beyond that of its simple use as a sword. Shinigami usually carry their Zanpakuto in the sealed state and activate the released forms as necessary.
All Zanpakuto have two levels of release. The first one is Shikai (initial release) and the second one is Bankai (final release). One must be able to do both of them to become a captain. The power and forms of Shikai and Bankai are dependent on the Zanpakuto. They also vary according to the wielder's strength and training.
Sealed
Most Zanpakuto look like regular katana, with slight variations between them as one would expect between different swords. There variations are commonly:
* Katana : The term katana may be applied to the standard-size, moderately-curved, Japanese sword with a blade length of greater than 60 cm (23.6 inches). The katana is characterized by its distinctive appearance: a curved, slender, single-edged blade, a circular or squared guard, and a hilt long enough to accommodate two hands.
* Wakizashi (literally "side insertion"): A short blade between 30 and 60 cm (12 and 24 inches), with an average of 50 cm (20 inches). It is similar to but shorter than a katana, and usually shorter than the kodachi (small sword). The wakizashi was usually worn together with the katana. When worn together the pair of swords were called daisho, which translates literally as "large and small". The katana was often called the long sword and the wakizashi the companion sword.
* Nodachi (literally "field sword"): The nodachi was more difficult to wield due to its size and weight. The length of the nodachi's hilt varied between twelve to thirteen inches (30 to 33 centimeters). Its cutting capability and range exceeds that of a katana, due to its weight and size.
In addition to the upgraded forms, the sealed form of a Zanpakuto can also be radically altered, though such alterations are purely cosmetic and usually don't offer any significant advantage in battle; one exception is Ikkaku Madarame's Hozukimaru, whose hilt stores a blood-clotting ointment. Zanpakuto can also be disguised as other objects, though such objects seem to need to be of similar proportions to the sword itself. Both Kisuke Urahara and General Yamamoto, for example, disguise their Zanpakuto as canes, Yamamoto's as a large walking stick that dissolves on command to reveal his sheathed Zanpakuto, Urahara's as a shikomizue, a thin blade hidden within the hollow shaft of a cane. A Zanpakuto will return to its sealed state when its wielder is knocked unconscious.
Shikai
The Shikai (Initial Release) is the second form (or first "upgraded" form) available to a Zanpakuto. To activate it, the Shinigami needs to learn the name of their Zanpakuto. This is not as easy as simply picking a name, as the living spirit of the Zanpakuto already has its own name. Therefore, the Shinigami must be able to communicate and harmonize with their Zanpakuto effectively, which requires being able to speak to the spirit within its world. A Zanpakuto can change size and shape depending on the spiritual strength of the owner.
Achievement of the Shikai is a mark of control of a Zanpakuto, and it appears to be a requirement for advancement within the Shinigami ranks, as presumably most seated officers are capable of Shikai. It is also required in order to achieve the rank of lieutenant; as it is a requirement most captains look for. After first learning the Zanpakuto's name, the Shikai can be performed at will simply by speaking a command followed by the Zanpakuto's name. The commands vary between users and range from a simple imperative verb to a short poem. They often relate to the Zanpakuto's signature ability, or hint at the nature of its spirit. This step may be bypassed by expert Shinigami who have learned how to use their Bankai.
Constant-Release: In rare cases, a Zanpakuto, once released, will remain in that state indefinitely. This is due to the vast amount of spiritual energy the possessor has that cannot be finely controlled upon initial generation of their Zanpakuto, as such the Zanpakuto is described as a constant released form. The only known Shinigami with Full-Time Release Zanpakuto are Ichigo Kurosaki and Kenpachi Zaraki. By the nature of this type of Zanpakuto, a Shinigami with a constant release form does not need a release command nor is it necessary to even know their Zanpakuto's name. However, not knowing the name of one's Zanpakuto can put a Shinigami at a disadvantage in terms of power against those who do know their Zanpakuto's name.
Shikai Forms: Once the Shikai is activated, the Zanpakuto will change form to access whatever special abilities it is capable of. They almost always take on the form of a weapon of some kind, though the type of weapon can vary greatly among different Zanpakuto. One exception is Retsu Unohana's Shikai, which seems to take on the characteristics of an animal, rather than a weapon.
Special Abilities: Once the Shikai is activated, the Zanpakuto will change form to access whatever special abilities it is capable of. The special abilities of the Zanpakuto, like the Zanpakuto itself, have specific names that usually must be announced to fully activate the effect. As Ichigo's Zanpakuto, Zangetsu, explains, knowing the name of an attack and not knowing it makes a very drastic difference in the amount of power that attack utilizes. Some special abilities are automatically activated upon release.
Bankai
The Bankai (Final Release) is the second and final upgraded form of a Zanpakuto. To achieve Bankai, one must be able to materialize and subjugate their Zanpakuto spirit. Materialization means the opposite of getting dragged into the Zanpakuto's inner world. Instead the wielder needs to summon the Zanpakuto'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 Zanpakuto. 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 Zanpakuto. 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 Zanpakuto 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 Zanpakuto's spirit, the Zanpakuto 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 Zanpakuto'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 Zanpakuto (for example Ichigo's Zangetsu becomes Tensa Zangetsu, Byakuya's Senbonzakura becomes Senbonzakura Kageyoshi). Certain Shinigami also change with their Zanpakuto, gaining clothing similar to what their Zanpakuto's spirit form looks like. Most Zanpakuto 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 Tensho 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 (Jizo) 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.
Sourced from
http://bleach.wikia.com/wiki/Zanpakut%C5%8D
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