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).

However, this offered merely a temporary solution to the menace; eventually, in the event of his death, the seal would dissolve and the Ten-Tailed Beast would once again be unleashed upon the world. Knowing this, the Sage devoted the remainder of his life to searching for a more permanent solution. Ultimately, the Sage succeeded by dividing up the chakra of the Ten-Tails into the nine monstrous entities that would later become known as "tailed beasts" and scattering them throughout the world. As a final precaution the Sage, using his Chibaku Tensei jutsu, entombed the physical remains of the Ten-Tails within a 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 the form of its body seemed to be closest to that of the Nine-Tailed Demon Fox's body. Also, it appeared to have only one eye, which had four Rinnegan-like concentric circles, with nine Sharingan-like tomoe in the three circles closest to the pupil.