Summoning: Impure World Reincarnation

This summoning is a forbidden technique used to revive the dead. The ninja will first sacrifice a living person to provide the vessel that the dead soul will inhabit. The dead body will then be encased in ash and dirt to resemble the original body of the person that was resurrected. When the ninja wishes to recall the souls, a portal will open in the ground and the caskets holding the bodies will appear. When the caskets open the bodies will be gray and in a state of minor decay, requiring a special fuda tag to restore their appearance. At this point, the revived person remembers their former lives, and thus is not under the user's control. The same tag used to restore their appearance also wipes the soul of its free will, putting them under the user's control. This technique can be used to summon multiple people at once, depending on the amount of sacrifices and the chakra level of the user.

Once complete, the revived is impossible to be killed 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.

Kabuto mentioned that the Second Hokage 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.

When Orochimaru used this jutsu, he sacrificed Kin Tsuchi and Zaku Abumi to revive Hashirama Senju and Tobirama Senju. Kabuto used it to resurrect Itachi Uchiha, Sasori, Deidara, Kakuzu and Nagato simultaneously. He also summoned a sixth coffin with an unseen person in it, whose presence forced Madara to accept Kabuto's offer for an alliance.

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.