Where does zanpaktou come from ?

Each Zanpakutō is a manifestation of its owner's soul, and has a unique name that a Shinigami is required to learn in order to use its Shikai and Bankai. As such, each Zanpakutō has unique abilities that match their owner's characteristics and personality, and their names usually hint to what these powers may be.

A Zanpakutō spirit is part of its owner's soul, and they 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.[1] 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. The spirit of a Zanpakutō dwells within an internal world, most naturally the wielder's soul, which is different for each Shinigami. The Zanpakutō spirit can normally only be seen by its wielder.

Though rare, it is not impossible for more than one soul to be attuned to the same spirit. In such cases, ancient law of the Soul Society will not allow two beings to wield the same Zanpakutō, in which case, they must duel for ownership of the spirit, with the winner gaining it and the loser being sentenced to death.

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.[2] Shinigami with vast 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, effectively, be forced to wield swords the size of skyscrapers.[3]

As stated, a Zanpakutō is a being unto itself and tends to be highly temperamental. Its mood is determined by how it is treated by its Shinigami as well as its overall personality. This allows a Shinigami to partially determine whether it will achieve its true power or not. For example, Yumichika Ayasegawa normally releases his Zanpakutō with the name Fuji Kujaku (Wisteria Peacock), even though its true name is Ruri'iro Kujaku (Azure Peacock), knowing that its favorite color is azure and that its least favorite is wisteria purple. He does this so that its true power is not seen. Since his Zanpakutō hates the nickname, it becomes moody and only releases a fraction of its power. This allows him to pass off his Zanpakutō as a simple melee-type rather than what it truly is, a Kidō-type, something that is unacceptable in the 11th Division due to an unspoken rule that all Zanpakutō in the 11th must be melee-types. Only when its true name is uttered is its full potential achieved.[4]

A Shinigami may find it hard to achieve its Zanpakutō's Shikai or Bankai due to its personality, which is a reflection of his or her own personality. Communication is key as seen evident with Kenpachi Zaraki, who has little or no communication with his Zanpakutō and therefore doesn't know its name, making its true potential not realized. This inability causes his power to damage his Zanpakutō rather than allowing it to truly help him.

Each Zanpakutō has the power to send Pluses to the Soul Society, purify Hollows, and transform the Zanpakutō itself into more elaborate and powerful forms. Not all Shinigami can hear the name of their Zanpakutō and thus can not achieve the higher tiers of power. In the anime-exclusive Zanpakutō Unknown Tales arc, it is revealed that every Zanpakutō has an instinct. This is both good and bad; while it grants them a great amount of power, those with the right abilities can awaken this instinct and use it to manipulate them, essentially controlling their minds. This is how Muramasa is controlling the Zanpakutō of Soul Society.

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. A Zanpakutō will return to its sealed state when its wielder is knocked unconscious.[5]

Sealed

Most Zanpakutō look like regular katana, with slight variations between them as one would expect between different swords. There variations are commonly:

* Katana (刀): 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 grip long enough to accommodate two hands.

* Wakizashi (脇差 (わきざし); meaning "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 (小太刀 (こだち); meaning "small sword"). The wakizashi was usually worn together with the katana. When worn together the pair of swords were called daishō, which translates literally as "large and small". The katana was often called the long sword and the wakizashi the companion sword.

* Nodachi (野太刀 (のだち); meaning "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 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.[6] 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, Yamamoto's as a large walking stick that dissolves on command to reveal his sheathed Zanpakutō, Urahara's as a shikomizue, a thin blade hidden within the hollow shaft of a cane.

Shinigami who are unable to communicate with their Zanpakutō, wield a nameless version known as Asauchi (浅打ち; literally, "shallow hit"), which refers to the grossly-reduced power of a Zanpakutō whose wielder cannot connect with it properly.[7]
Shikai

The Shikai (始解, Initial Release) is the second form (or 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 and harmonize 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 presumably most seated officers are capable of Shikai. It is also required in order to achieve the rank of lieutenant; it is a requirement most captains look for. 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. In rare cases, a Zanpakutō, 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 Zanpakutō, as such the Zanpakutō is described as a constant released form. The only known Shinigami with Full-Time Release Zanpakutō are Ichigo Kurosaki and Kenpachi Zaraki. By the nature of this type of Zanpakutō, a Shinigami with a constant release form doesn't need a release command nor is it necessary to even know their Zanpakutō's name. However, not knowing the name of one's zanpakutō can put you at a disadvantage in terms of power against those who do know their Zanpakutō's name.

Once the Shikai is activated, the Zanpakutō will change form to access whatever special abilities it is capable of. 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 very drastic difference in the amount of power that attack utilises.
Bankai

The Bankai (卍解, 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 100 or more years of combat experience beyond attaining Shikai for even the strongest Shinigami to achieve, it is rare and almost exclusively limited to the thirteen Captains in the Gotei 13.[8]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.[9] 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.[8]

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 tends to be similar to the Shikai's ability/theme but amplifies it to much greater 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.