Summoning: Impure World Reincarnation

The is a forbidden technique where, using a living person as a vessel, a deceased person's soul can be called back to the world of the living and bound to it. The vessel will then take on the form that person had in life, thereby reincarnating them to do their summoner's bidding.

Background
It was originally developed by Tobirama Senju. Madara Uchiha claimed it to be intended for the dead to be reincarnated into soldiers able to clear a battlefield with devastating and otherwise suicidal attacks. The technique was learned by Orochimaru who modified it for his ends. Orochimaru's aide Kabuto Yakushi, claiming that this technique to be the greatest and most powerful technique in the shinobi universe due to it posing no risk at all to the user, and mastered it to a greater degree than the previous users.

To perform this technique, the user must first acquire some of the DNA of the person they intend to reincarnate. Kabuto remarks that this basically amounts to grave-robbing, although blood stains or organs salvaged after the target's death also work. The soul of the intended reincarnated must also reside in the ; for example, those whose soul has been consumed by the Death God cannot be reincarnated. However, if it is a case where the souls are freed from inside the Shinigami, then a user of this technique is free to reincarnate them once again.

Next, a living sacrifice is required for the soul of the reincarnated to use as a vessel. Once all prerequisites for the technique have been met, the acquired DNA of the person is smeared on a special scroll and once the scroll is activated, the remains spread out in the form of a special seal with the living sacrifice in the centre. Then dust and ash encase the sacrifice's body, giving them the same appearance that the reincarnated had at the time of their death. The process is apparently somewhat painful as seen when Fū was used to reincarnate Torune. The person is then reincarnated and the end product is usually stored in a casket until summoned by the user. The user can theoretically reincarnate a limitless number of people in this way, so long as they have enough sacrifices, chakra and DNA to perform the technique. Despite this, this technique also has limitations, such as the individual having limitations that inhibited them in life such as Nagato's lack of mobility and depending on the summoner's mastery of the technique, the reincarnated individuals might not be brought back with the full power they had in life.

Summoning
When first summoned, the body of the reincarnated is stored in a coffin; their body will appear grey and in a state of minor "decay": marred by cracks and other imperfections, and the individual will also appear to be "disengaged". Upon awakening they retain their personality, memories, and all abilities they had in life, including kekkei genkai, kekkei tōta and summoning contracts. Although clothing is also retained by the reincarnated shinobi, weaponry on the other hand is not and must be procured by the user when necessary, as seen when two members of the Seven Ninja Swordsmen of the Mist were unable to retrieve their weapons from a scroll upon reincarnation, as they were currently in the possession of others. However, once obtained, shinobi that have stored their weapons in or about their person are still able to re-summon their tools, regardless of the number of times they are stored within their respective coffin. As a downside, the reincarnated seem to retain any permanent body damage and physical limitations they received during their lifetime. The summoned all have grey sclerae and retain their original eye colours, however, the entire eye is darkened. When Kabuto takes complete control of them however, they gain black sclerae and white irides. The appearance of a reincarnated dōjutsu wielder's eye doesn't change at all. The summoned can be recalled at will by the user, causing an empty coffin to emerge and reclaim the individual, and the summoner can also use reincarnated individuals to remotely summon coffins containing additional reinforcements.

Along with this, the summoned individual can feel the person's chakra flowing through them. Shinobi with sensory ability are also able to go one step further and decipher the actual source of the chakra that is manipulating them, even if the summoner is within the confines of a barrier. This poses a potential disadvantage to the summoner, as a renegade summon that is no longer under their direct control may be able to locate them.

Control
After the individual has been summoned and awakened, the summoner can control the individual by using a special talisman, which is attached to the end of a kunai and implanted in the brain. Upon implanting the talisman, it is safe for the summoner to complete the technique and allow the summoned ones to regenerate further, eliminating any remaining imperfections with the exception of small cracks that may run across the body and restoring colour to their bodies.

Different talismans can be used to enact different degrees of control. Orochimaru's talismans completely eliminated the personalities of the summoned, turning them into mindless killing machines under his command. Kabuto prefers to use a weaker controlling talisman on certain individuals, allowing them to retain some degree of their own personality. The latter method is useful for psychological warfare, and can also allow the summoned individual to tap into their own emotions, memories, or creativity to enhance their effectiveness in combat, but also carries a host of drawbacks; it takes more effort to retain control with the weaker binding which can make it possible for the hearts of the summoned to be swayed by strong emotions, which can result in the soul breaking free and returning to the Pure World, or the reincarnated contradicting orders that they are given. Reincarnated shinobi with their personalities still intact have their movements limited to certain actions, and are basically on "auto-pilot" to react to enemy techniques. Once they recognise an enemy technique their bodies are programmed to counter it with an appropriate method.

Even so, Kabuto can completely remove their personalities if he needed, and may elect to if they resist his command too strongly. After gaining more experience with the technique, and absorbing Orochimaru's chakra from Anko Mitarashi, Kabuto exerts greater control over the summoned and can even overwrite his own talismans remotely, greatly increasing the power of the binding. Once deployed, he can either remotely control or track the summoned individuals using pebbles he manipulates on a simple grid, which acts as a map of sorts. When a reincarnated person is sealed, the pebble cracks and disappears, and the reincarnated shinobi can no longer be used to summon others. This was seen when Haku and Zabuza were immobilised by Kakashi, Maki, and the rest of their division. The pebble for his trump card was black in colour while the others are light-coloured. The summoner can also directly talk through the summoned individual.

Provided the summoner has not eliminated the reincarnated person's personality, by doing certain handseals they can break the link binding him or her to the Impure World Reincarnation contract. In doing so, they take full control of their being and if the technique is cancelled and contract is now their own, they will remain under their own power, with the immortal body and chakra the technique provides.

Enhancements
Unlike while they were still alive, the reincarnated have unlimited chakra, as well as seemingly near-unlimited stamina and can fight continuously, as neither Kimimaro nor Itachi showed any fatigue from their illnesses as they did in life, and Itachi was even able to use multiple Mangekyō Sharingan techniques without showing exhaustion, while minor uses caused fatigue in life.

The reincarnated are impossible to kill by normal means. Any damage they receive will easily regenerate, be it lost limbs or complete disintegration. Not even killing the summoner will get rid of them. The summoned can make practical use of the this regeneration ability by using suicide attacks and large scale ninjutsu to wipe out everything on a battlefield and then simply regenerate.

The reincarnated can also be modified. While the technique usually reincarnates one as they where upon death, the reincarnated can also have various points from their life combined to get the best possible results as seen with Madara Uchiha, having been brought back "beyond his prime" despite dying an withered old man. In addition to regaining his peak health and combat prowess, he also retained the Wood Release and Rinnegan he never had in his youth. Further evidence of the selective modifications were seen from Nagato and Madara both retaining the Rinnegan in each eyes when reincarnated at the same time and despite Obito currently using one of them as well.

Tobi also modified the eyes of the reincarnated jinchūriki to match his own, though this was only due to the use of the Six Paths of Pain technique which caused his eyes to be reflected in theirs, as well as resealing the tailed beasts back into their respective hosts.

Countering
While the reincarnated are immune to traditional attacks such as weapon-based attacks and even the death of the summoner being ineffective, there are three discovered methods to end the technique:
 * 1) Seal away the soul of the reincarnated individual.
 * 2) Have the summoner end the technique. As they are unlikely to do this voluntarily, using a genjutsu to trick them into cancelling the technique is ideal. The sequence of hand seals for cancelling the technique is Rat → Ox → Monkey → Tiger → Dragon → Boar and saying  (when demonstrating the technique to Tobi, Kabuto lied that the seals to release it are Dog → Horse → Tiger).   If the summoner dies before the technique is deactivated, however, then this method is no longer a viable option.
 * 3) The reincarnated individual is affected in some emotional way that gives them closure; Kankurō notes that human emotions are not so easily restrained. However, this method cannot work if the summoner eradicates the personality of the reincarnated individual before their soul is freed of its bonds.

Once any of the first three methods are accomplished, the reincarnated individual will briefly regain their personalities before crumbling back into dust, leaving the bodies of the sacrificed individuals lying among the ashes. If the technique is cancelled by the user, the reincarnated's bodies are surrounded by a light before beginning to deconstruct. After this, the reincarnated individual's soul will begin to ascend, leaving behind the corpse of their vessel.

Aside from the three methods of actually defeating the technique outright, there are four discovered methods of countering the effectiveness of this technique:
 * 1) The technique can be partially combated by immobilising the bodies in a way that it cannot move, act, or be recalled by the summoner. Although this does not truly defeat the summoning, it is effective at neutralising the immediate threat the reincarnated pose in battle.
 * 2) If the reincarnated still retain their personalities, they can speak against the user or themselves by providing hints and advice to the opponents, even in the midst of involuntarily fighting them.  With considerable strength of will, the summoner's authority can even be completely overridden, albeit briefly.
 * 3) Itachi Uchiha was able to use Kotoamatsukami to layer another order on top of the one established by this technique overriding the user's control. This can be remedied if the user can place a stronger binding talisman into the said reincarnated person, thus re-establishing control.
 * 4) What Madara describes as "the one risk of the technique", if the reincarnated knows the specific seal, one can release the Impure World Reincarnation's summoning contract, severing the bond between themselves and the summoner. This leaves the individual effectively immortal with an unlimited amount of chakra, and the summoner has no control over the individual's mind and movements any longer, nor can the technique be deactivated on the said individual.

It should be noted that the reincarnated are still vulnerable to any technique that could affect them while they were alive, but they are able to reform as soon as they receive any damage. Furthermore, while the reincarnated can't be destroyed and feel no pain which originates from bodily harm or mutilation,    they are still vulnerable to the drawbacks from their own techniques or weaknesses. Such examples include Hanzō's poison acting as a paralysis agent, Mū along with the Second Mizukage being left weakened after using certain techniques, and Itachi's eye bleeding on the activation of his Mangekyō Sharingan.

Other Known Reincarnated Shinobi
In the anime, it was shown that Kabuto has reincarnated a great deal more shinobi than shown in the manga. While most of these have yet to be seen on the battlefield, the ones that have been used are as follow:
 * In the Chikara Arc, during the invasion of the Tonika Village and the subsequent skirmish with officers from the Hachō Village and Team Kakashi, Kabuto summoned several shinobi:
 * A burly Kumogakure shinobi wielding a bō.
 * A Sunagakure shinobi wielding a pair of chakram.
 * An Iwagakure shinobi wearing a flame-thrower device.
 * A Kirigakure shinobi wielding a sword.
 * A Takigakure shinobi with an eye-patch covering his left eye.
 * Kabuto would later reincarnate an unknown Kumo-nin who wore a bird-motif mask which covered the upper-half of his face and a Suna-nin who wore his flak jacket open and wielded a broad axe. They were assigned to a team with Hayate Gekkō to retrieve the KIA list — a scroll which had been used to seal away the bodies of shinobi with kekkei genkai and other special abilities.

Trivia

 * is what Japanese Buddhists call the normal world, unenlightened people live in. The current, living world. The world of the people who have not yet have escaped the polluting thoughts (greed, hatred, delusion, etc.) that result in suffering.
 * A possible translation of Tensei is "Resurrection". In terms of this technique, both "Reincarnation" and "Resurrection" can be found; the sacrificed body is reincarnated into the body of the summoned shinobi, while the reincarnated shinobi is resurrected from the dead.
 * In chapter 117 of the manga, when Orochimaru first performs the technique, he tries to summon a third coffin. When the coffin is not summoned, he comments on its failure. Years later, in chapter 520, Kabuto explains that Orochimaru's attempt to summon the Fourth Hokage had failed due to his soul not residing in the "Pure World", which Orochimaru was not aware of. In episode 69 of the anime, when Orochimaru used this technique, the coffin briefly appears, with written on it; though the summoning of the Fourth Hokage still fails. Based on Kabuto's demonstration, it would seem the actual failure would have occurred when Orochimaru performed the original preparatory ritual.
 * The video game, Naruto: Ultimate Ninja 2, the (non-canon) storyline contradicts the idea the soul of the reincarnated must reside in the Pure World, as Orochimaru reincarnates the Third Hokage no differently than anyone else he summons in the game.
 * Kabuto also mentions his desire to reincarnate certain shinobi, including:
 * Jiraiya, though his body lies at the bottom of the ocean where the water pressure is too great to reach. However, he claimed that the bloodstains on the Six Paths of Pain's weapons may provide enough DNA.
 * Shisui Uchiha, but Kabuto could not find his body, though he suggested that Shisui's crushed eye, which is in Tobi's possession, may be sufficient. Later, after learning that Shisui's other eye was on the battlefield, he seemed excited at the prospect of obtaining it. However, Itachi incinerated it.
 * Originally, in the manga, when the First and Second Hokage were reincarnated, their eyes appeared blank. In the anime version, their eyes appeared normal. Later in Part II of the anime and manga, the sclera of a reincarnated shinobi is changed to dark grey.
 * In Naruto Shippūden: Ultimate Ninja Storm Generations, Kabuto Yakushi (Snake Cloak Form) uses this as his ultimate technique. He summons Kakuzu, Itachi and Deidara, who attack the opponent using the Lightning Release: False Darkness, Fire Release: Great Fireball Technique and C1 techniques respectively.
 * In the anime, when Kabuto summons the reincarnated shinobi for war, there are several additional ones who do not appear in the manga. Also when eradicating the personality of Hanzō, he use several hand seals instead of just one.