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. Tobi'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. The astonishing feat of not only successfully sealing the Ten-Tails, but also suppressing it, made the Sage a deified hero throughout the shinobi world.

Appearance
In the manga, the Ten-Tails has only been seen in silhouette, but the form of its body seems to be closest to Kurama's body, but with several spiky protrusions visible behind its head. Also, it appears to have only one unique eye, which has four Rinnegan-like concentric circles, with nine Sharingan-like tomoe in the three circles closest to the pupil coloured red.

Abilities
As the original form of all nine tailed beasts, the Ten-Tails was said to possess massive and overwhelmingly foul chakra that is potent enough to bring total despair and destruction to the world. According to Kurama, not only is the Ten-Tails the source of all chakra, but is also the progenitor of everything that exists in this world. Kurama even admitted that the Ten-Tails, even if it didn't fully obtain all of Kurama and Gyūki's chakra, would be too much for the fox to take on alone and stated that the Sage feared that the Ten-Tails' revival would result in the end of the world. Because the nine tailed beasts are merely by-products of the Ten-Tails, it is presumed that the Ten-Tails to have all their abilities at its disposal. It was shown having a single unique eye, but what it is or if it is a unique dōjutsu is unknown.

Trivia

 * In the anime, the Ten-Tails was first shown with a red eye with tomoe, 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 but was missing the tomoe. Its figure and eye were corrected in episode 253.
 * was a yōkai of titanic proportions in Japanese mythology. His size and strength were immeasurable, and the creature was said to be so enormous that his footprints created hollows, ponds, and lakes, and that his strength was to blame for the current placement of mountains and rivers. In one legend, Daidarabotchi wanted to see which was heavier, Mount Fuji or Mount Tsukuba. He weighed them on a great scale. However, when he was finished, he dropped the latter, splitting its peak in two.