Rinnegan

The Rinnegan (輪廻眼; Literally meaning "Saṃsāra Eye") is reputed as the most exalted eyes amongst the Three Great Dōjutsu (三大瞳術, San Daidōjutsu; Literally meaning "Three Great Eye Techniques"). It is characterised by its ripple-like pattern which spreads over the eyeballs, with light purple irides and sclerae.

Background
It was admired as being able to bring forth all ninjutsu. It was said that in times when the world is in disorder, a person is sent down from the heavens to become either a "God of Creation" who will calm the world, or a "Destroyer" who will reduce everything to nothingness. The Rinnegan allows the user to use the Six Paths Technique.

The first person to possess the Rinnegan was the Sage of the Six Paths. Because of the Sage's mythical status, many people believed the Rinnegan was only a legend or a mutation. Because the Rinnegan was originally wielded by the Sage of the Six Paths, re-merging his bloodlines: that of the Senju and Uchiha can lead to that person awakening this dōjutsu. This was proven when Madara Uchiha later awakened the dōjutsu by infusing Hashirama Senju's DNA into his body; however it did not manifest until he was near the end of his natural lifespan. At some point after this, he would implant his eyes into Nagato — a young Senju descendant. Obito Uchiha, being the partner of and given the guise of Madara, believes that since it was given to Nagato by Madara, he is entitled to taking the Rinnegan from Nagato's body after his death. He implants one of the Rinnegan in his left socket, replacing the Sharingan he sacrificed.

When Madara was reincarnated by Kabuto Yakushi to fight the Allied Shinobi Forces, Madara once again gained access to the dōjutsu; his Eternal Mangekyō Sharingan transformed into the Rinnegan. Based on data collected over the years, Kabuto Yakushi and Orochimaru theorised that because the Sharingan originated from the Rinnegan, it was possible for the dōjutsu to change into the Rinnegan as part of a "natural evolution". This is supported by the fact he maintains his Susanoo while having his Rinnegan activated. Upon reincarnation, Nagato also retained the Rinnegan.

Abilities
One who gains the Rinnegan has access to a multitude of abilities. On its own, the Rinnegan allows the user to see chakra, including otherwise hidden barriers such as the one which surrounded Konohagakure. The holder of these eyes have the potential to completely master all five basic nature transformations that form the basis of ninjutsu. It can also decipher more of the Sage of the Six Paths' tablet than the Sharingan and the Mangekyō Sharingan are able to. The Rinnegan can also allow its user to summon and control the Demonic Statue of the Outer Path.

One of the Rinnegan's trademark techniques is the Six Paths Technique, which grants the user multiple abilities, known as Paths. The Animal Path can summon several different creatures. The Naraka Path can summon the King of Hell, which could be used for interrogation and restoration. The Preta Path can absorb any chakra-based ninjutsu. The Asura Path can clad the user in mechanised armour and grant them various weapons. The Human Path can read minds, at the cost of ripping out the victim's soul. The Deva Path allows the manipulation of gravity-based forces to either repel or pull in an enemy.

Finally the user is granted the Outer Path, also known as the seventh Path. It granted the user control over life and death. It can be used to control tailed beasts in a similar fashion to the Sharingan and the Wood Release with its chakra chains. The Outer Path also granted a shared field of vision between its user and their summoned creatures and Six Paths of Pain, which gave multiple angles of vision and neutralised any blind spots. At this point all of their eyes would also sport the Rinnegan.

When he was crippled by Hanzō, Nagato developed the Six Paths of Pain where he uses his Rinnegan and chakra receivers on freshly killed corpses to serve as extensions of himself. By doing so, Nagato could see each body's field of vision simultaneously, coordinating them with extremely precise planning. Nagato would also channel one of the abilities of the Six Paths Technique through each corpse. Obito also applied this technique, though he used reincarnated jinchūriki. Their eyes sported the Sharingan and Rinnegan, in identical fashion to Obito, and had their tailed beasts temporarily re-sealed inside them, though the Uchiha would not have them use the powers of the Six Paths Technique.

While the powers of the Rinnegan remain intact upon implanting them in a new person, like Nagato and Obito Uchiha, only one who "naturally" manifests the Rinnegan, as in Madara Uchiha's case, is able to switch between the Rinnegan and Sharingan, otherwise, the Rinnegan is always activated.