Rinnegan 輪廻眼 and Sage of the Six Paths (六道仙人, Rikudō Sennin)

Rinnengan:
The Rinnegan (輪廻眼; Literally meaning "Samsara Eye") is said to be the most revered and powerful of all the three great dōjutsu. The Rinnegan's occurrence is so rare that Jiraiya once believed its existence was only a legend, and Kakashi Hatake believes it to be only a mutation.^[1] There have thus far been only two known users: the Sage of the Six Paths and Nagato. With the Sage long deceased, and Nagato now dead there are no other confirmed Rinnegan users alive at this point.
The Rinnegan is characterized by a ripple-like pattern around the pupil, with a grayish-purple iris and sclera. In addition to enabling the user to quickly master various jutsu, the user may also use all six forms of chakra nature transformation which, as noted by Ibiki Morino, literally allows its user to use any technique they wish. It also allows the user to use all of the six paths techniques, a series of ninjutsu unique to the Rinnegan user.^[2] Like the Sharingan and the Byakugan, the Rinnegan also allows the user to see chakra, including otherwise hidden barriers. According to Nagato's partner Konan, the possessor of the Rinnegan can also control the seventh path of Pain, who is "beyond life and death", that can use the Outer Path: Samsara of Heavenly Life Technique, which allows the Rinnegan user to revive all people who have recently died.
Because Nagato's Six Paths of Pain were all reanimated corpses, their Rinnegan was added to their bodies by Nagato through some unknown means. By doing so, Nagato could see each individual body's field of vision, coordinating the bodies with extremely precise planning, even during battles. Due to this effect, unlike the Byakugan, the Rinnegan does not have a blind spot, as long as the bodies are looking in every direction or at least one body is looking at the desired target. Because of this advantage, an opponent must blind one body, or distract all others in order for a blind spot to be opened. The animals summoned by the Animal Path also possessed the Rinnegan, suggesting that Nagato controlled and could see through them as well. Madara Uchiha also stated that while the Sharingan and more with the Mangekyō Sharingan can translate some of the text on the tablet the Sage left behind, only the Rinnegan can fully read the words on it.
It is later revealed that Madara has become interested in obtaining the Rinnegan to prepare for war, and for his own ambitions.^[3] However, Konan wisely forsaw this and hid Nagato's remains.^[4]

Sage of the Six Paths (六道仙人, Rikudō Sennin)

The Sage of the Six Paths (六道仙人, Rikudō Sennin) was said to be the legendary figure who founded the ninja world and was the creator and first user of modern ninjutsu (essentially making him the first shinobi, in the modern context of the term), as well as the first jinchūriki. 

Background 

According to Jiraiya, the Sage was a priest who appeared long ago in an age of endless war. He was the first to explain the truth about chakra and tried to use his power and wisdom to lead the world to peace instead of war. The Sage travelled throughout the world, spreading his ideas and religion, the Shinobi Sect (忍宗, Ninshū), which eventually came to be known as ninjutsu. His great deeds and true desire to bring peace to the war-torn world led him to become widely known as the Saviour of this World (この世の救世主, Kono Yo no Kyūseishu).

The Sage of the Six Paths dividing the Ten-Tails into the nine tailed beasts.

According to Madara Uchiha, centuries before the beginning of the main story line the Sage defeated the most powerful demon in the ninja world, the Ten-Tailed Beast. In order to protect the world from the Ten-Tails, the Sage developed a new technique. The Sage was able to use this technique to seal the beast into his own body, allowing him to harness and control its terrifying power and making himself the first jinchūriki. Though he had successfully sealed the Ten-Tails, its power was so great that the seal would break after the Sage's death. Aware of this, the Sage used the rest of his strength to divide the beast's power into nine parts, which would become known as the tailed beasts. He then used the Chibaku Tensei to seal the Ten-Tails' remains and send them into the sky where no one could ever reach them, thus creating the moon. The Sage would leave behind a tablet for his descendants which details his history with the Ten-Tails. Only those who possess the Rinnegan can fully decipher what is written on the tablet, while a person with a different dōjutsu can still partially interpret the information.
The Sage's ultimate wish to establish peace throughout the world was a goal that could never be achieved within his lifetime alone. Knowing that his end was approaching, the Sage entrusted his strength, dream, and legacy to his two sons. The older son, who inherited the Sage's "eyes": the power of his chakra and his spiritual energy, believed that power was the true key to peace. The younger son, who inherited the Sage's "body"; his willpower and physical energy, believed that love was the true key to peace.
On his deathbed, the Sage chose his younger son to be his successor. Overcome by bitterness and envy, the older son attacked the younger, beginning a war between them. Their feud would continue on through their descendants: the Senju clan and the Uchiha clan.^[1]
Centuries later, the Rinnegan itself would resurface in Nagato. Nagato's connection to the Sage has yet to be explained, although Jiraiya believed that the Sage could have somehow been "reborn" as Nagato in the present era. Nagato himself, however, admitted that his own power paled in comparison to that of the Sage.^[2] Still others, such as Kakashi Hatake, believed that the Sage is merely a myth, and that the Rinnegan is merely a mutation.^[3] However, the Nine-Tailed Demon Fox's recognition of the Sage (and the tailed beasts' apparent status as the only survivors from the Sage's lifetime), lends credibility to his existence.^[4]

Appearance 

Although depicted only as a silhouette, several details can be inferred about the Sage's appearance. He had long, spiky hair flowing backwards, in a style similar to Madara Uchiha's. He wore what appeared to be a cloak with a high open collar or armor similar to that worn by Hashirama Senju, showing his necklace with six red magatama, similar to the one worn by Nagato's Deva Path. He seemed to have had horns on his forehead; this could have been a forehead protector similar to the one worn by Jiraiya or, if the outline of Naruto's new seal is an indication, it might have been part of his hairstyle. 

Abilities 

The Sage was shown carrying two objects. One was what appeared to be a large katana in his hand, whilst the other was a shakujō, a Buddhist ringed staff which is meant to be used in prayer, but can also be effectively employed as a weapon. 

Pioneer Ninja 

The Sage was revered as the man to first understand the very nature of chakra itself. This wisdom, combined with his kekkai genkai, the Rinnegan, allowed the Sage to create the first modern ninjutsu. With such powers at his disposal the Sage obtained a huge following, as men and women from across the world strived to learn from his teachings. As the Sage's disciples grew continuously greater in number it led to the creation of ninja clans, and later, ninja villages. It is impossible to overestimate the Sage's effect on the course of the ninja world's development. As the first true ninja, he created the ninja world, revolutionized it, and left it forever changed. 

Rinnegan 

The Sage was said to have been the only shinobi to have fully mastered the Rinnegan, allowing him to manipulate all six types of nature chakra. He is revered as one of the most legendary figures in the Naruto universe, and is most likely the most powerful ninja that ever existed. He was so powerful that he created the moon using Chibaku Tensei. Even Nagato stated that his power paled in comparison to that of the Sage. 

Jinchūriki 

The Sage developed the first jutsu capable of sealing a tailed beast inside a human body, making himself the first jinchūriki in history. Having the Ten-Tails' power contained within him, the Sage developed immeasurable power and chakra that far surpassed all of his peers. Even on his deathbed, the Sage was able to employ his Chibaku Tensei technique on a massive scale and use that chakra to create the moon and seal the body of the Ten-Tails within it, a clear testament to his incredible power.