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Kidō (鬼道, "demon way", or "spirit way" sometimes translated as Demon Arts; Viz "Soul Reaper's powers"; "Spells") is the technique that Shinigami use to focus their Spiritual Energy into a wide variety of magic spells that can serve many purposes, such as defense or combat. According to Sōsuke Aizen, it is one of the four basic Shinigami combat forms.

The Kidō Corps is a branch of Soul Society's military which excels in the mastery of Kidō.

Kidō Mechanics
Each spell is graded on a scale from 1 to 100, the latter being the most powerful and the most difficult to perform. There can't be two or more spells with the same number. To use Kidō, a Shinigami must recite the specific incantation for the spell, which is often long and requires a few seconds to speak.

This weakness can be negated by Shinigami capable of using Eishōhaki (詠唱破棄, lit. "Incantation Abandonment"), a technique that releases Kidō without a spirit chant. While it decreases the time needed to release the Kidō, it drastically weakens the spell. While anyone well versed in Kidō can use this technique, it is most effective when done by a skilled practitioner, otherwise the spell could utterly fail. The most skilled experts are capable of using this technique to amazing effect.

Another method of circumventing the necessity, of uttering an incantation for a Kidō, is something called Kōjutsu Eishō (後述詠唱, lit. "Later Mention Incantation"), where the incantation is recited after initiating the spell to power it up. So far, only Hachigen Ushōda has been seen using this technique, in order to reinforce the nature of his Ryūbi Jōmon during his fight against Baraggan Luisenbarn.

Another technique to avoid the incantation is called Tanshuku Eishō (短縮詠唱, lit. '"Shortened Incantation'"), where you skip some lines of the incantation to save some time. But the disadvantage is that the spell is then weakened. Not as much as in Eishōhaki but still weakened.

The power of a spell relies on the power of the user, as even a low-level spell can be utterly devastating when utilized by a high-class Shinigami.

Kidō can also be used for practical purposes as well. Renji Abarai has demonstrated that a kidō spell can be used to light a dark hallway if the spell is tweaked in the right way.

Kidō Types
There are two main types of Kidō: binding spells and destructive spells.
 * Bakudō (縛道, lit. "Way of Binding"): These are a broad category of defensive spells which blocks/repels attacks or freezes enemies in place.
 * Hadō (破道, lit. "Way of Destruction"): These are offensive spells which harm an enemy or cause damage to objects.

Kidō Combat
Though Kidō is commonly used by Shinigami in combat, it has several weaknesses that make it difficult to use for that purpose. All of the spells have fairly long incantations and require concentration; because of this, the person using the Kidō leaves themselves open to attack during the duration of the incantation.

In addition, if an opponent is familiar with Kidō, there is the possibility of them identifying the spell by its incantation before it can even be used. However, highly skilled Kidō users can use various techniques to counter these weaknesses, such as Eishōhaki, Tanshuku Eishō, Kōjutsu Eishō and Nijū Eishō (二重詠唱, lit. "Double Incantation"), the latter involves mixing the incantations of two or more Kidō spells together to save time and confuse the opponent.

A rarer use of Kidō is mixing various spells (such as Bakudō and Hadō together) for unique effects, which is the mark of a skilled user. Only a few Kidō can be tweaked to

Kidō is commonly seen used in instances of surprise attacks, rather then all-out assaults, being most effective when one's opponent doesn't see it coming. Kidō is also usable with just the spell name and no incantation or spell number.

Bakudō
Bakudō (縛道, Way of Binding) is a category of spells used for defense, binding, capture, healing, camouflage, sealing, escape, tracking, communication and traveling.

Hadō
Hadō (破道, Way of Destruction) is category of spells used for offense, harming, damaging and destruction.

Unnumbered Spells
These spells are not explicitly stated to be in one category or the other and have no known number. However, it seems that most of these unnumbered spells are binding spells.

Non-Spells

 * Shunkō (瞬閧, Flash Cry): An advanced secret battle technique that combines the grappling technique of Flash_Cry.jpga and the incantation technique of Kidō. It is an ultra-powerful technique in which one drives Kidō into one's arms and legs. The Kidō itself can also be controlled and fired at opponents from the body. Shunkō first appeared being used by Captain Suì-Fēng in her fight against her old master, Yoruichi Shihōin. Suì-Fēng states she invented the technique of combining Kidō and Hakuda so recently that she had no experience with it in battle, let alone enough time to give it a name. Yoruichi Shihōin, however, reveals Shunkō is the technique's name, and that she invented and learned the ability long before Suì-Fēng. The executive attire has no back or sleeves because the fabric will rip apart when the user's back and arms are filled with Kidō. Though Yoruichi Shihōin's Shunkō is perfected, unlike Suì-Fēng's version, she still has difficulties controlling it. It could be said that Shunkō is a secret technique used by the Corps Commander of the Executive Militia. The uniform worn when punishment is carried out is specifically geared for Shunkō.
 * Hanki (反鬼, Reverse Demon): An ability which nullifies an opponent's Kidō by hitting it with another one of perfectly opposite speed and energy. The ability can even neutralize an opponent's movements. (Somewhat erroneously named Hanki Sōsai in the Viz translation, as Sōsai (相殺) means "Mutual Cancellation", referring to the ability's effect.)

Kidō-based Weapons

 * Kidōhō (鬼道砲, Kidō Cannon) — In Bleach: Memories of Nobody, the Kidō Cannon is a high-intensity Kidō_Cannon.jpgy weapon powered by Kidō. It consists of a crystalline focus point attached to the top of a massive pole, with several smaller crystal structures circling up towards the main one. It is powered by hundreds of Shinigami working in concert, creating an energy blast capable of obliterating a small dimension. The blast forms into several animal shapes which roar just before contact. The power of the blast is such that, in addition to destroying its target, it also causes a small amount of damage to both the Human World and Soul Society. Despite its power, it is not a reusable weapon, as it burns into a pile of scrap after being fired.
 * Kidō Gun — This weapon was developed by the Bount creator, Ran'Tao. It is similar to the Kidō Cannon;Kidō_Gun.jpg however, unlike the Kidōhō, which can only be fired once, this weapon holds more resemblance in functionality to a shotgun. It fires concentrated spheres of spiritual energy that can be quite harmful, as it was capable of severely injuring Jin Kariya. It is unknown as to where the Kidō Gun's energy source is derived from.

Trivia

 * In the English dub of the anime, "Kidō" was originally mispronounced as if it was spelled "Kaidō."