Toward the end of the series, Sasuke revealed his plan to bare all of the world's hatred and establish lasting world peace. Naruto pointed out that even if he were to succeed today, Sasuke's plan isn't a permanent solution because he would eventually die. Sasuke countered by explaining that with the power he has, there are several ways for him to live forever. That statement wasn't wrong either. There are a surprising number of ways to achieve immortality in Naruto.
Numerous characters throughout the series have discovered some method of immortality. Some versions of immortality are more effective than others. Each version has its own pros and cons that come with it. Most of the individuals that achieved immortality belonged to the Akatsuki at one point or another. Let's discuss each immortal in Naruto and how their particular brand of immortality functions.
Orochimaru's quest for immortality is probably the most well-known. As a young child, Orochimaru became obsessed with learning forbidden Jutsu. It eventually became his goal to learn every Jutsu in the world. But he soon realized that with his current body, that would be impossible. There wasn't enough time in a normal human lifespan, and there were particular techniques his body couldn't perform. To solve this problem, Orochimaru decided to become immortal.
Orochimaru actually developed two forms of immortality. The first method was his Immortality Jutsu. This technique allowed Orochimaru to transfer his soul into another body. Once the ritual was complete, Orochimaru would then have access to all the abilities of that body while still retaining all of his own knowledge and techniques. As long as Orochimaru could continue to take over new bodies, he would live forever. The downside to this Jutsu is that Orochimaru could only perform it once every three years. Also once those three years are up, his current body begins to rapidly decay, forcing him to find a new host.
The other method that Orochimaru developed was his Cursed Seal. Orochimaru placed a Cursed Seal on many of his subordinates. If they survived the application and learned to master it the Cursed Seal granted them a major power boost. Each Cursed Seal contained a piece of Orochimaru's chakra and soul inside of it. This allowed him allowed him to enact a limited amount of control over them. Because each seal contains a piece of Orochimaru's soul if he was killed he could be revived by releasing the seal and allowing his soul to inhabit a suitable body. Sasuke did this by using a piece of Kabuto's flesh, which contained Orochimaru's DNA and releasing Anko's Cursed Seal. The major con to this method of immorality is that it required other people to be willing to revive Orochimaru.
Sasori believed that the ultimate form of art was one that never changed or decayed. To better embody his ideal version of art, Sasori replaced his body with that of a puppet. As a puppet, Sasori no longer aged, grew tired, or required nutrition. He had turned himself into what he believed to be the perfect form of art while achieving immortality in the process.
Although Sasori's puppet body would allow him to live forever, there were some severe downsides. Sasori's body cannot feel for instance. This can be beneficial at times since he can't feel pain, but that also means he can't experience any form of touch, including pleasure. Sasori's puppet body also still required chakra to function, which is why he had his living core of flesh implemented in the body. This core was also Sasori's most vulnerable spot and his greatest weakness.
Kakuzu was one of the oldest living ninjas in the series. He was alive during the founding of the Hidden Leaf Village and had survived an assassination attempt against the First Hokage. Kakuzu was scorned by his village for failing his mission though. This disdain drove him to abandon the village but not before killing village elders and stealing their greatest, forbidden Jutsu.
The Jutsu he stole was Earth Grudge Fear. This technique allowed Kakuzu to control black threads. These threads can reattach appendages almost instantly. The most important part of this ability is that Kakuza can use these threads to remove the beating hearts of his victims and add them to his own body. Doing this extends his life span, but if all the additional hearts are destroyed, Kakuzu dies as normal.
Kakuzu's partner in the Akatsuki also had a form of immortality. Hidan likely has the strongest version of immortality out of all those listed so far. Due to extensive experimentation and religious rituals, Hidan can survive almost any mortal wound. Not even decapitation can kill Hidan. This immortality also allows him to perform a Jutsu that reflects damage inflicted on himself onto his target. The only way that Hidan can naturally die is through malnutrition.
The biggest downside to Hidan's immorality is that he can't instantly regenerate from wounds or lost limbs like Orochimaru. This means that Hidan can easily be incapacitated by removing his limbs. Shikamaru took advantage of this fact by blowing off Hidan's limbs and trapping him in a pit. Hidan was left to starve to death, which is a pretty gruesome way to go.
Impure World Reincarnation
During the Fourth Shinobi War, almost every deceased Akatsuki member was revived using Impure World Reincarnation and forced to fight in the war. These revived bodies also had a form of immorality. They could quickly regenerate from any damage they took and possessed infinite chakra. These revived shinobi were a massive advantage for the Akatsuki during the war.
The revived shinobi were compelled to do the bidding of the person who resurrected them. When Kabuto used these ninjas in the war he often forced them to fight against their loved ones as a form of mental warfare. The only way to defeat these immortal ninjas was to seal their bodies or force the caster of the Jutsu to release them. Madara revealed that if the revived ninja knew the hand seals for the Jutsu they could break free of the caster's control and receive all of the benefits of the Jutsu without almost none of the setbacks.
The White Zetsu used by Madara and the Akatuski were created when Kaguya Ōtsutsuki tethered her subjects to the God Tree. Being connected to the God Tree for years turned these individuals into powerful soldiers loyal to Kaguya. She planned to use these White Zetsu to form an army to fight the rest of her clan, who were also immortal beings.
The White Zetsu seemingly do not age. Even after hundreds of years since their creation, the White Zetsu army did not grow older or lose their strength. These soldiers also possessed incredible regenerative abilities. Their cells have been used to heal heavily injured bodies and grow new appendages. Those that have been turned into White Zetsu lose their original identity permanently. The White Zetsu appeared to lack a will of their own and follow the commands of those in charge without hesitation. Many characters in Naruto often fantasize about obtaining immortality, but more often than not, the negatives heavily out weight the positives.