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 possesses neither ideals nor 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.

Appearance
Silhouetted prior to its introduction in the story line, the Ten-Tails is a cyclops-like humanoid in appearance with a conch shell-like upper body with long, spiked-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 and mentioned to be an immature stage in its development, the Ten-Tails' form was considerably different. Foremost, 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, as well as a spiked chin. 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. Also, no legs of any sort can be seen. The endings of its tails look like leaf buds and have been shown able to unfurl. Despite being incomplete, the Ten-Tails is still much larger than Gyūki and Kurama, so much that when Gyūki got close to it, it was around the size of the Ten-Tails' hand.

After reaching maturity, the Ten-Tails gains a humanoid silhouette, almost similar to that of the Demonic Statue and the one shown in the flashbacks, but its bodily proportions are ridiculously elongated and emaciated and it retains the spiked bulb from its previous form on its back. It's left arm is also missing at the elbow. The head changes heavily, with one ear on the right, the eye on the center, and the mouth on the left in a similar fashion to the Asura Path's head. There are also horns on its head, and the tails' ends turn out be hands.

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 told them that its revival would result in the end of the world. 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 unmeasurable.

As a tailed beast, it is able to create a Tailed Beast Ball, firing it in a beam-like fashion, rather than an actual sphere. It is immensely powerful as it was capable of overpowering the multiple Tailed Beast Balls from Gyūki and Kurama without losing momentum; all while still destroying six of Kurama's tails and causing a trail of destruction across the landscape. In its mature form, its Tailed Beast Ball takes a unique, cone-like shape and gains an immense increase in range and destructive power, enough to destroy entire cities or the Allied Forces HQ with a single blast each, despite them being miles away.

Despite being in an incomplete state, the Ten-Tails possess great physical strength and is remarkably fast, despite its immense size as it was able to attack and repel both Kurama and Gyūki before any of them could react. It could even casually flick back a Tailed Beast Ball with one of its fingers. The beast also has an extreme level of durability as it was unfazed from sustaining massive damage. A combined Tailed Beast Ball from Naruto and Killer B didn't even harm it. A single roar from the Ten-Tails was strong enough to tear up the landscape around it. In its mature form its tails become elongated arms, strong enough to easily break through the restraints created by the Allied Forces.

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 prepared 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, cutting short the opposition's declaration of victory. With this, Obito echoed Kakashi's sentiments that it was in fact the end, of this world and not the battle. Elsewhere, though disappointed that he was not able to capture the Eight and Nine-Tails to complete the beast's revival, Madara casted this aside and noted that they needed to get on with the plan.

With the revival complete, Obito and Madara situated themselves atop the beast, from which two tendrils manifested and connected themselves to the two Uchiha. Not wanting to waste any time, Obito noted that he wanted to cast the Infinite Tsukuyomi immediately while Madara wanted to test the beast's power first. As the beast attacked Gyūki and Kurama, it quickly landed a blow on Kurama before the fox had time to retaliate. Landing on either side of the beast, Kurama and Gyūki launched a barrage of simultaneous Tailed Beast Balls at the beast only for the Ten-Tails to counter using one of its own, completely nullifying the oncoming attacks. After the dust cleared, a Naruto shadow clone and Kakashi appeared above the Ten-Tails where Kakashi released Gyūki who was preparing to launch a point-blank range Tailed Beast Ball.

Merely flicking the ball back at Gyūki with a finger, the beast prepares to crush Kakashi and Naruto beneath its tail, but only manages to dispel the shadow clone as Kakashi was pushed to safety. Afterwards, the Uchiha have the Ten-Tails prepare a Tailed Beast Ball to decimate their severely exhausted opponents but much to everyone's shock, it missed. It is revealed that control of the beast had been momentarily wrested from their control. With the battlefield now shrouded in mist and all sensory skills being jammed, the question of whether they thought they were hiding was raised. With a single swipe of its tail, the beast is able to dissolve the mist. Just then the Wind Release: Wind Cutter Technique is used to counter, and when the mist cleared, it is revealed that the rest of the Allied Shinobi Forces had joined the fray.

The Alliance launched a barrage of techniques in order to incapacitate the Ten-Tails, so that they could get to Obito and Madara. As the beast is subsequently immobilised in what was effectively cement, the two Uchiha commented that the effort is futile as the Ten-Tails began to mature. It burts forth from its prison with an even more grotesque form and proceeds to destroys several towns. Among these is the shinobi headquarters and everyone in it.

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.