Summoning: Impure World Reincarnation is a kinjutsu created by Tobirama Senju and subsequently improved by Orochimaru, and then almost perfected by Kabuto Yakushi. The technique binds the soul of a deceased person to a living vessel, restoring the body to the state it was in at the last moments of life, and forcing the summoned to serve the summoner's will. To perform it, three components are necessary: a DNA sample of the target, a living sacrifice, and a soul residing in the Afterlife.
After activation, the genetic material is applied to a special scroll, and a ritual seal forms around the sacrifice. The sacrifice's body becomes covered in ash and dust, taking on the appearance of the resurrected; the process is agonizing, and the vessel's personality is completely displaced by the summoned's chakra. In Tobirama's original execution, the souls returned greatly weakened, and only a few bodies could be controlled. Orochimaru improved the power, but the true breakthrough was achieved by Kabuto: the summoned are almost equal to their living selves in strength, and the number that can be controlled simultaneously is limited only by available sacrifices and the user's own power. This turns the technique into the ultimate weapon.
Control is exercised through a talisman implanted in the brain. At a low level of control, the summoned retains their personality and tactical freedom, but must obey. At the highest level, the personality is erased, and every movement is dictated directly, which limits multitasking. The summoned possess an immortal body: any wounds regenerate, chakra replenishes endlessly, and damage such as severed limbs does not prevent continued combat and soon regenerates just the same. At the same time, memories of life, kekkei genkai, and all mastered techniques are preserved. The technique does not reproduce illnesses and does not return foreign fragments if they were not part of the body at the time of death.
The Impure World Reincarnation can only be undone in three ways: extracting the soul (e.g., with the God of Death Seal), forcing the summoner to release the technique (often through genjutsu), or giving the summoned an emotional closure that breaks the contract. The summoned can also free themselves if they know the release seals, becoming a completely independent immortal.
