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, depending on the amount of live sacrifices and chakra that's needed to summon them.

The revived are impossible to kill by normal means. If a limb is destroyed or removed, it will simply regenerate. 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.