Zanpakutō

A  is the main weapon of the  Shinigami, the Arrancar and the  Vizard. The art of using a zanpakutō is called.

Overview
Most zanpakutō look like regular katana, with slight variations between them as one would expect between different swords. Each zanpakutō is imbued with a unique sentient spirit, which gives the sword the power to send pluses to the afterlife, purify Hollows, and transform the zanpakutō itself into more elaborate and powerful forms. To facilitate these tasks, zanpakutō have unique abilities that match their owner's characteristics.

A zanpakutō spirit is part of its owner's soul, and often share the same personality traits. At the same time, they are also independent, and thus can be considered a Shinigami's partner in battle. Because they are part of their owner's soul, a zanpakutō cannot be replaced, though it will slowly regenerate if broken. As a Shinigami gains power, their zanpakutō can change form by allowing the inner being to manifest itself to a greater degree in the outside world. The sword and the inner being are synonymous to each other. No matter what form (or size) a zanpakutō takes, it is always virtually effortless for its owner to wield because it is a part of its owner's soul. On the other hand, it is much more difficult for a Shinigami to wield a zanpakutō that is not their own; Kaname Tōsen is the only known Shinigami so far to actually do such a thing.

Forms
Depending upon the Shinigami's ability to communicate with and control their zanpakutō's spirit, a zanpakutō 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 zanpakutō, giving it abilities far beyond that of its simple use as a sword. Shinigami usually carry their zanpakutō in the sealed state and activate the released forms as necessary. Shinigami with zanpakutō of the constant-release type, such as Ichigo Kurosaki, are unable to seal their zanpakutō and therefore carry them in released form constantly.

In addition to the upgraded forms, the sealed form of a zanpakutō 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 Hōzukimaru, whose hilt stores a blood-clotting ointment. Shinigami with great spiritual power and who are unable to control it have larger zanpakutō, as Ichigo Kurosaki did when he first became a Shinigami. Isshin Kurosaki, Ichigo's father, explains at one point that high-level Shinigami must be capable of maintaining and suppressing the size of their sword. Otherwise, they would end up swinging around swords the size of skyscrapers. Zanpakutō 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 zanpakutō as canes.

Shinigami who are unable to communicate with their zanpakutō wield a nameless version known as asauchi.

Shikai
The is the first "upgraded" form available to a zanpakutō. To activate it, the Shinigami needs to learn the name of their zanpakutō. This is not as easy as simply picking a name, as the living spirit of the zanpakutō already has its own name. Therefore, the Shinigami must be able to communicate with their zanpakutō effectively, which requires being able to speak to the spirit within its world. Achievement of the shikai is a mark of control of a zanpakutō, and it appears to be a requirement for advancement within the Shinigami ranks, as most (with the notable exception of Kenpachi Zaraki) seated officers and presumably all lieutenants are capable of shikai.

After first learning the zanpakutō's name, the shikai can be performed at will simply by speaking a command followed by the zanpakutō's name. The commands vary between users and range from a simple imperative verb to a short poem. They often relate to the zanpakutō'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, although this has only been demonstrated once, by Renji Abarai. In rare cases, a zanpakutō, once released, will remain in that state indefinitely. Such a zanpakutō is described as a full-time released form type, such as Ichigo's.

Once the shikai is activated, the zanpakutō will change form to access whatever special abilities it is capable of. They usually remain in the form of a sword (with multiple exceptions, such as Soifon wielding a stinger, Ikkaku Madarame wielding a spear, Kaien Shiba wielding a trident, Marechiyo Ōmaeda wielding a spiked ball and Shūhei Hisagi wielding two connected dual scythes), but the type of sword differs radically among various zanpakutō. The special abilities of the zanpakutō, like the zanpakutō itself, have specific names that usually must be announced to fully activate the effect. As Ichigo's zanpakutō, Zangetsu, explains, knowing the name of an attack and not knowing it makes a difference in the amount of power that attack has.

Bankai
The is the second and final 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 of ten. Because such an act usually takes hundreds of 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.

Types
There are several types of zanpakutō, classed by the main purpose or effect of their special abilities. For example, Ichigo Kurosaki's zanpakutō is a melee-type zanpakutō, since it is primarily designed for close-combat, though it does also have ranged attacks, such as the Getsuga Tensho. Yumichika Ayasegawa's zanpakutō, on the other hand, is a kidō-type zanpakutō, since it's designed around its special ability rather than its close combat potential.

Some zanpakutō are also classified by element: Tōshirō Hitsugaya and Rukia Kuchiki possess ice-type zanpakutō, Shigekuni Yamamoto-Genryūsai and Shūsuke Amagai use fire-type zanpakutō, and the zanpakutō of Kaien Shiba and Jūshirō Ukitake use water-based abilities.

Another type of zanpakutō is known as the constant-release type. Actually a "sub-type" of zanpakutō classification, this group pertains to zanpakutō whose wielders are unable to revert them back to their default sealed states after initiating the shikai release. Such a phenomenon is mainly due to the vast spiritual power of the sword's wielder, such that they are unable to completely seal their zanpakutō like other Shinigami. Only two Shinigami are known to have a constant-release type: Ichigo Kurosaki and Kenpachi Zaraki, both of which have tremendous spiritual power, exceeding that of a normal Captain-class Shinigami.

Though not mentioned, it does seem that bankai makes the effect of the shikai special attacks stronger. The power of Ichigo's Getsuga Tenshō, for example, increases when bankai is used.

Then there is the difference between the single bladed shikai version of a zanpakutō, and the dual bladed version. Both Captain Shunsui Kyōraku and Jūshirō Ukitake have a shikai with dual blades. In the fight between the two Captains and Shigekuni Yamamoto-Genryūsai Yamamoto notes that dual bladed zanpakutō are rare. Shūhei Hisagi also reveals to have a dual bladed zanpakutō in his fight against the arrancar Findor.

Resurrección
Arrancar, who are hollows that have removed their masks and gained Shinigami powers, have zanpakutō and can perform something similar to a shinigami's release called resurrección (Spanish for resurrection). Unlike a Shinigami's zanpakutō, however, an Arrancar's zanpakutō is simply the arrancar's Hollow powers sealed into sword form. By releasing their zanpakutō in the same manner as a Shinigami, an Arrancar can release his or her Hollow powers.

Because their powers are sealed in their zanpakutō, an Arrancar's release will usually transform some or all of their body into a Hollow-like form, allowing them to use their abilities freely. This form usually reflects what the arrancar looked like as a Hollow, although to highly varying degrees. Arrancar normally only return to their human form when they reseal their hollow powers; doing so without resealing, though possible, is a permanent change comparable to amputating a limb. The reason Arrancar go through such trouble to seal their power is twofold: it allows them to maintain human form and conserve their power in times of inactivity.