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 ambition, which was created 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 seal 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.

Personality
As noted by Kurama, the Ten-Tails does not possess ideals or feelings and as such cannot be sensed through such means as those that involve detecting negative feelings or malevolence. The fox went on to liken the beast to energy flowing around a planet or what one would feel from the earth or the sea — an innate, natural force. When Naruto Uzumaki attempted to use Sage Mode to gauge the power of the beast, he was overwhelmed by the vastness of the beast's power, which Kurama noted as 'it cannot be measured'.

Appearance
Silhouetted prior to its introduction in the story line, the Ten-Tails is cyclops-like humanoid in appearance with a conch shell-like upper body and spike-like protrusions on its back. Its unique eye, taking up most of its head, 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, resembling a conch shell, and its tails were bushier. When revived, due to being imperfect its form was considerably different. Foremostly, the Ten-Tails has six tomoe in its eye instead of the full nine. The beast also has a wide, cavernous mouth with multiple rows of sharp, pointed teeth. It now possesses a grotesquely bloated, round body in place of its previous humanoid silhouette, and seems to crawl on its belly, while the branch-like spiky protrusions grow out of its back at a straight angle. Its hind legs, if present, have yet to be seen. Despite being incomplete, The Ten-Tails is still much larger than the Nine-Tails and Eight-Tails.

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 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. Despite being in an incomplete state, a combined Tailed Beast Ball from Naruto and Killer B didn't even damage it. Despite a multiple tailed beast ball being fired at it by Kurama and Gyuki, Madara stated they need not even have to dodge it.

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.

With the revival complete, Madara opted to test its power to defeat the Eight-Tails and the Nine-Tails to ensure the smooth flow of the Eye of the Moon Plan. Controlling the consciousness of the beast, The Ten-Tails quickly landed a blow on Kurama before it had time to retaliate. Plain in sight, Kurama and Gyuki launch simultaneous Tailed Beast Balls at the beast only for the Ten-Tails to absorb and to return the attack with greater force causing a huge explosion. After the dust clears, a Naruto clone and Kakashi are shown atop the beast's head wherein Kakashi releases Kamui to release a prepared Tailed Beast Ball by the Eight-Tails to be hit at the beast at point blank range.

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 Daidarabotchi'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.