Rinnegan

The Rinnegan is considered to be the strongest doujutsu to ever exist and is one of the 3 Great Doujutsu.

Acquisition
The Rinnegan can be acquired by having a proper mix of the chakras of all the Clans, mainly the Hyuga and Uchiha clans. A person with all of these different chakras can then awaken their Rinnegan, which can take years to even manifest.

The Otsutsuki can naturally awaken the Rinnegan in their eyes, and even on their forehead and palms.

Abilities
The Rinnegan gives users a huge variety of abilities, even when deactivated.

Users of the Rinnegan can easily master and use all 5 nature transformations, create black receivers in which they can transmit their chakra, and immunity to ocular genjutsu.

They also have access to a family of abilities collectively known as the Six Paths Technique: A Rinnegan user will not gain all of the Paths at once, and often can only use them all through training and mastery of their Rinnegan.
 * Deva Path: Control over fundamental forces
 * Asura Path: Ability to mechanize one's body
 * Human Path: Ability to drain souls
 * Animal Path: Ability to summon powerful summons
 * Preta Path: Ability to drain chakra
 * Naraka Path: Ability to summon the King of Hell
 * Outer Path: Access to powerful abilities

The vision abilities of the Rinnegan depend on the "path" the Rinnegan developed from - the Sharingan, the Byakugan, or Pure (no prior doujutsu)

Rinnegans also seem to form a kind of special ability for each user.