Outer Path: Samsara of Heavenly Life Technique was the ultimate forbidden Ninjutsu innate to the Rinnegan. It could manipulate yin and yang, govern life and death, and truly resurrect the dead.
However, unlike Impure World Reincarnation, which had no side effects whatsoever, Outer Path: Samsara of Heavenly Life Technique had tremendous side effects, requiring the caster to expend vast amounts of Chakra and life force beyond their limits.
In the world of Naruto, two people had used this technique: Nagato and Obito Uchiha.
After Naruto persuaded him with his talk-no-jutsu, Nagato used his nearly exhausted body to resurrect the many people who had died because of his attack on Konoha. Obito Uchiha, meanwhile, had his body controlled by Black Zetsu after the Ten-Tails was extracted from him and he fell into utter despair, resurrecting Madara Uchiha.
It seemed like trading a life for a life, but "life" here actually meant life force rather than life itself. The best evidence was Yucheng, who had been in peak condition before casting this Ninjutsu: although his body became extremely weak afterward, it was not fatal.
Of course, the life force consumed by this technique would certainly have killed an ordinary human, leaving no chance of survival. But after Yucheng formed a contract with Apep, the elevation of his life allowed his lifespan to surpass human limits, and the life force contained within his body was also beyond that of ordinary people.
However, neither Ei nor Chiyo knew this. Ei, especially, had witnessed the miracle of Chiyo returning from the dead with her own eyes, and immediately panicked when she saw Yucheng's bloodless face. Even Chiyo, who had just been rejoicing in her resurrection, lost her smile at the sight and hurried to Yucheng's side.
Neither of them was mortal, and yet, throughout the