Summoning: Impure World Reincarnation

The Summoning: Impure World Resurrection is a forbidden technique used to revive the dead. It was originally developed by Tobirama Senju, who classified it as kinjutsu and never completed it, and later completed by Orochimaru. Kabuto now claims to have mastered it to a degree even higher than them. Kabuto also claims that the technique poses no risk to the user.

Conditions
To perform this technique, the user must first acquire some of the DNA of the person they intend to revive. 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 revived must also reside in the ; victims of the Dead Demon Consuming Seal, for example, cannot be resurrected. Next, a living sacrifice is required for the soul of the resurrected to use as a vessel.

Once all the requirements for the technique have been met, the physical remains acquired are smeared on a special scroll and once the scroll is activated, the remains spread out in the form of a special symbol with the living sacrifice in the center of the symbol. Then dust and ash encases the sacrifice's body, giving them the same appearance that the revived had at the time of their death. The person is then revived and the end product is usually stored in a casket until summoned by the user. The user can theoretically revive a limitless number of people in this way, so long as they have enough sacrifices and chakra to perform the technique.

Details
When first summoned, the body of the resurrected appears gray and in a state of minor decay. They retain their personality, memories, and all abilities they had in life, including kekkei genkai. They can be restored to a closer likeness of their original appearance by the application of a special tag. This tag can also remove their personalities, binding them to the user's will, but to what extent is up to the user. Kabuto prefers to exercise only minimal restraint over those he revives, but this is very demanding on him and it is often simpler to completely eradicate their personalities, which makes them easier to control. Kabuto can remotely control them through pebbles he manipulates on top of a map.

Countering
The revived are impossible to kill by normal means. Any damage they receive will regenerate, be it lost limbs or complete disintegration. Not even killing the summoner will get rid of them. There are only three known methods to defeat the technique: Once any of these methods is accomplished, the bodies turn back to ash and dust, with the bodies of the individuals sacrificed lying among the ashes.
 * 1) Seal away the soul of the revived individual.
 * 2) Have the summoner end the technique. As they are unlikely to do this voluntarily, using a genjutsu to trick them into canceling the technique is ideal. The sequence of hand seals for canceling the technique is Dog → Horse → Tiger and saying "release".
 * 3) The revived individual is affected in some emotional way that gives them closure.

Known resurrected
There have actually been even more summoned, for Kabuto had failures from when he accidentally summoned the wrong people due to sometimes being unable to tell which DNA samples belonged to different people.

Trivia

 * In episode #69 of the anime, when Orochimaru used this jutsu against Hiruzen, three coffins were summoned, with the third coffin had written on it. This coffin was not shown in chapter #117 (although Orochimaru tried to summon three coffins) however, years later in chapter #520, Kabuto mentioned that Orochimaru's attempt to summon the Fourth Hokage had failed due to his soul not residing in the "pure world".
 * When Kabuto first approached Madara about joining forces, he summoned Deidara, Itachi, Kakuzu, Nagato, and Sasori as a demonstration of power. When Madara threatened to refuse Kabuto's offer, Kabuto summoned a sixth casket that compelled Madara to not only join forces with Kabuto, but to call him a "Madman!". The contents of the sixth casket were obscured.
 * The video game, Naruto: Ultimate Ninja 2, the (non-canon) storyline contradicts the idea the soul of the revived must reside in the Pure World, as Orochimaru revives the Third Hokage no differently then anyone else he summons in the game.