Ten-Tails

The is the original, primordial demon of the series. All nine of the tailed beasts are but portions of chakra divided from the Ten-Tails by the Sage of the Six Paths. The beast is considered to be the progenitor of the world as well as a signal of the beginning of the end of the world, should it ever be revived. Obito Uchiha's ultimate goal, which was inspired and designed by Madara Uchiha, 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 and then 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 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 Creation of All Things ability to separate the Ten-Tails' chakra from its body and divide it into nine separate constructs 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.

Centuries later, after the Rinnegan would once again manifest itself in one of the sage's descendants: Madara Uchiha, a method to summon the empty vessel of the Ten-Tails called the Demonic Statue of the Outer Path was realised. This ability would later be passed on to Nagato of Amegakure, and later Obito as well.

Appearance
Apart from knowing what the beast's vessel looks like, the complete Ten-Tails has only been seen in silhouette, but the form of its body seems humanoid, but with several spiky protrusions visible behind its head. It has one unique eye, which has four concentric circles, with nine tomoe in the three circles closest to the pupil. When seen in a flashback, the Ten-Tails appearance was a bit more "crass". The spiky protrusions on its back were much higher and its tails were bushier.

When revived in the present, the Ten-Tails has the skin texture of the Demonic Statue of the Outer Path. Its upper body resembles a spiked conch shell and its eye takes up most of its head. However, having been revived in an incomplete form, its eye has just seven tomoe instead of the full nine.

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 stated that it was a god that created countries and had the power to swallow oceans, split the land, and carry mountains. In addition, Kurama admitted that it alone wouldn't even stand a chance against the Ten-Tails, and stated that the Sage feared that its revival would result in the end of the world. As a tailed beast, it is presumably able to create a Tailed Beast Ball. It was shown having a single unique eye, but what it is or if it is a unique dōjutsu is unknown.

Shinobi World War Arc
During their battle with Obito and Madara Uchiha, Naruto and Killer B prepare a massive Tailed Beast Ball in order to destroy the Demonic Statue of the Outer Path before the Ten-Tails could be fully revived. Though the statue's chakra vanished after the attack, once the smoke settled, it was revealed that the beast had successfully been revived. With this, Obito declared the end of this world and Madara notes that they should get on with the plan, though he seemed disappointed that he had not captured the Eight and Nine-Tails.

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.
 * Deidarabotchi, also known as, was a yōkai of titanic proportions in Japanese mythology. Kurama's explanation of how powerful the Ten-Tails is reflects Deidarabotchi's role in the formation of Japan's wilderness, such as the two peaks of Mount Tsukuba, according to Japanese myth.
 * The name "Datara" may refer to, a one-eyed, one-legged blacksmith said to live deep in the mountains of Kumano. Myths vary on his nature as a yōkai or the fallen deity Ameno-me-Hitotsu. Either way, the legends say of Ippon-datara's ability to forge living things from metal that makes another reference to the Ten-Tails as the progenitor of the world.
 * The Ten-Tails can be revived without fully collecting all nine tailed-beasts, having a fraction of the chakra of which is enough to revive it in an incomplete form.