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 Eye of the Moon Plan, is to capture and merge all nine of the tailed beasts back into the Ten-Tailed Beast, and become its host to cast the reflection of his Sharingan on the moon, casting a genjustsu 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-Tailed Beast. Once captured, he sealed the Ten-Tails into his own body, becoming the first jinchūriki. However, the Sage knew that if and when he were to die that the Ten-Tailed Beast would not die with him. Without the Sage's own chakra to keep the Ten-Tails suppressed, (similar to Sasuke's chakra keeping Orochimaru suppressed,) the beast's foul and powerful chakra would break through the seal, thus releasing it to wreak havoc upon the world once again.

Reaching the end of his life, the Sage of the Six Paths sought a solution. He used his remaining time to divide up the chakra of the Ten-Tails into nine separate, monstrous constructs of living chakra, banishing them all to the far reaches of the world. These nine constructs of living chakra would become known throughout the world as the tailed beasts. As a further measure against its return, the Sage used Chibaku Tensei to seal the actual physical body of the Ten-Tailed Beast within a prison of compressed stone and hurled it far up into the heavens, creating the moon.

Appearance
So far, the Ten-Tailed Beast 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 also has only one eye. This single eye has four Rinnegan-like concentric circles with nine Sharingan-like tomoe in the three circles closest to the pupil.