Summoning: Impure World Reincarnation

Summoning: Impure World Resurrection 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 seal 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 seal used to restore their appearance also wipes the soul of its free will, putting them under the user's control. This technique can also 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. When the Third fought against the revived First and Second Hokage, he had to create shadow clones and perform the Dead Demon Consuming Seal on both of them to kill them. In doing so, the bodies turned back to ash and dust, with the bodies of the individuals sacrificed to lay among the ashes.

Trivia

 * When Orochimaru used this jutsu against Hiruzen, three coffins were summoned, but Hiruzen stops one of them. This third coffin is not seen in the manga, but in the anime it has (a reference to the 4th Hokage) written on it.
 * Orochimaru used Kin Tsuchi and Zaku Abumi as sacrifices to use this jutsu.