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, the Rinnegan would once again manifest itself in one of the sage's descendants: Madara Uchiha. With his new power, Madara summoned the empty vessel of the Ten-Tails back into the world and named it the Demonic Statue of the Outer Path. Joined by Obito Uchiha, who continued his work after his natural death, Madara devised a means to reunite the Demonic Statue and the tailed beasts back into their one original form so he can use the Ten-Tails to enact his life-long goal: the Eye of the Moon Plan.

Personality
As noted by Kurama, the Ten-Tails possesses neither feelings nor ideals, 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.

Due to being incomplete in its revival, with six tomoe in its eye instead of nine, the Ten-Tails appeared in what Madara and Obito revealed to be its larval stage. In its immature form, the Ten-Tails possessed a grotesque bulb-like body much larger than Gyūki and Kurama, with spiky protrusions growing out of its back, and a wide, cavernous mouth with multiple rows of sharp, pointed teeth, as well as a spiked chin. It had long arms and clawed hands, but no visible hind-legs. It also had dark veins similar to those of a plant all over its body, and the endings of its tails resembled leaf buds, which had been shown to unfurl as the beast matured.

After further maturing, the Ten-Tails more closely resembles its humanoid silhouette from the flashback. Its bodily proportions are very elongated and emaciated with a twisting neck, yet it retains the spiked bulb from its previous form on its back. Its left arm is also missing at the elbow. Its facial appearance also changes greatly, growing a single ear on the right side of its head while the mouth moves to the left side as the teeth become straight. There are also horns on its head. The monster's tails now end with hands similar to the chakra arms formed by other tailed beasts.

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 immeasurable. This was again reaffirmed when the beast's chakra completely warped the shape of the Sensing Water Ball in the Allied Forces HQ in both its initial and second forms.

As a tailed beast, it is able to create a Tailed Beast Ball and fire it in a beam-like fashion, rather than an actual sphere. It is immensely powerful as it was capable of overpowering 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 second form, its Tailed Beast Ball takes a unique, cone-like shape and gains an immense increase in range, firing speed and destructive power, enough to destroy mountain ranges, 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 possesses immense physical strength and is remarkably fast, despite its tremendous 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 second form, its tails become elongated arms, which are strong enough to break free from the various restraints created by the Allied Forces. The beast also managed to easily smash through Kitsuchi's massive Earth Release: Sandwich Technique.

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 its metamorphosis into its mature form. Once fully matured, the Ten-Tails bursts forth from its prison and proceeds to devastate several towns, with the shinobi headquarters and everyone in it targeted as well. After wiping out the headquarters, Madara realises that Ten-Tails is becoming more troublesome to control as he allows Obito to assume command as only he seal the monster into Madara's body. Once in control, Obito uses the Ten-Tails to channel the Wood Release: Cutting Technique to decimate the Allied Forces with wooden stakes, with Neji Hyūga among the fatally wounded, before it is sandwiched between Kitsuchi's Earth Release: Sandwich Technique.

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. However, even without the other tailed beasts the Ten-Tails will slowly become its former self, though it takes time for it to "mature" into its complete form, making it harder for Madara and Obito to control it as it does.
 * The Ten-Tails mature form's head has three primary features; an eye, an ear and a mouth. This is possibly a reference to the phrase "see no evil, hear no evil, speak no evil".