Summoning: Impure World Reincarnation

This summoning is a forbidden technique used to revive the dead. Kabuto Yakushi mentions that Tobirama Senju and Orochimaru were the only ones who knew how to use this technique, and Kabuto now claims to have mastered it to a degree even higher than them. Before the technique can be performed, a living sacrifice must be found. The sacrifice's body is used as a vessel for the soul of whoever is being revived, with dirt and ash encasing them to resemble the original body of the person being resurrected. The end product is stored in a casket until summoned by the user.

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. However, with the application of a special tag their personalities can be removed, with the extent of this removal being entirely dependent on the individual structure of the particular inserted tag, but whatever the psychological implications their appearance is restored to a closer likeness of how they looked in life. This technique can be used to summon multiple people at once, dependent upon the possession of the necessary amount of live sacrifices and chakra that's needed to summon them. However; as the summoner approaches the limit of the quantity of individuals they can resurrect concurrently, the difficultly in restraining each of the subjects own will becomes increasingly more taxing on the user, often forcing them to resort to the eradication of the personalities of the deceased in order to effectively maintain control.

The revived are impossible to kill by normal means. If a limb is destroyed or removed, it will simply regenerate, and unlike normal summonings the Impure World Resurrection summons can't be dispersed even by killing the user. To kill the revived, the soul has to be removed. Once this is accomplished, the bodies turn back to ash and dust, with the bodies of the individuals sacrificed lying among the ashes.

Trivia

 * When Orochimaru used this jutsu against Hiruzen, three coffins were summoned, but Hiruzen stopped one of them. This third coffin is not seen in the manga, but in the anime it has written on it.
 * 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 unseen contents of a casket which worried the user's opposition connects Orochimaru and Kabuto's summoning. In the former case it was a third casket which Hiruzen Sarutobi stopped from opening and in the latter the contents of the casket were obscured from view but appeared to put Madara on edge.