Ten-Tails

The is the original demon of the Naruto universe. All nine of the tailed beasts are but portions of chakra divided from the Ten-Tails. Madara Uchiha's ultimate goal, the Moon's Eye Plan, is to capture and merge all nine of the tailed beasts back into the Ten-Tails, and become its host to cast the reflection of his Sharingan on the moon, casting a genjutsu known as the Infinite Tsukuyomi which will put the entire world under his control.

Background
Centuries before the founding of the hidden villages, the Sage of the Six Paths saved the world from the Ten-Tails. He managed to subdue the beast and sealed its chakra within his own body, becoming the first jinchūriki; using his own chakra to suppress the beast (similar to how Sasuke used his chakra to suppress Orochimaru's).

This offered only a temporary solution to the menace in that the seal would eventually dissolve in the event of his death, so the Sage devoted the remainder of his life to a more permanent method. He succeeded by ultimately dividing up the chakra of the Ten-Tails into the nine monstrous entities that would later become known as "tailed beasts" and scattered them throughout the world. As a secondary precaution, the Sage, using the Chibaku Tensei jutsu, entombed the physical remains of the Ten-Tails within the colossal stone prison and hurled it into the heavens, creating the moon.

Appearance
So far, the Ten-Tails has only been seen once, in silhouette, but its body seems to be closest to the form of the Nine-Tailed Demon Fox's body. It appeared to have only one eye, for it has four Rinnegan-like concentric circles with nine Sharingan-like tomoe in the three circles closest to the pupil.