Ten-Tails

The is the original, primordial 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 Eye of the Moon Plan, is to capture and merge all nine of the tailed beasts back into the Ten-Tails, and become its jinchūriki 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 it within his own body, becoming what could be considered the first jinchūriki. However, this offered merely a temporary solution to the menace; in the event of his death, the seal would dissolve and the Ten-Tails would once again be unleashed upon the world. Knowing this, the Sage searched for a more permanent solution. Ultimately, the Sage succeeded by using his ability to divide the Ten-Tails' chakra into the nine monstrous entities that would later become known as "tailed beasts", scattering them throughout the world. As a final precaution, using Chibaku Tensei, the Sage 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 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 unique eye, which had four Rinnegan-like concentric circles, with nine Sharingan-like tomoe in the three circles closest to the pupil colored red. Also, it seems to have several spiky protrusions on its back.

Abilities
As the fusion of all nine tailed beasts, the Ten-Tails is said to possess massive and overwhelmingly foul chakra that is potent enough to bring total despair and destruction to the world. Because the nine tailed beasts are merely by-products of the Ten-Tails, it is presumed that the Ten-Tails has all of their traits, condensed into one body. It was shown having a single unique eye, but what it is or if it is a unique dōjutsu is unknown.

Trivia

 * The astonishing feat of not only successfully sealing the Ten-Tails, but also suppressing it, made the Sage a god-like hero throughout the shinobi world.
 * In the anime, The Ten-Tails was first shown with a red eye, like the Sharingan during its battle with the Sage of Six Paths, then when it was about to be sealed into the moon, its eye was purple like the Rinnegan. This is probably because of the Sage's influence.