Ten-Tailed Beast

The Ten-Tailed Beast is a gigantic demonic deity of unfathomable power from the Naruto franchise, who presided over the world in the beginning of time. Long ago it was originally the God-Tree known as the Shinju (神樹, English: Divine Tree), but after a woman named Kaguya Otsutsuki possessed it, her corrupted mind transformed the divine tree into the demonic Ten-Tailed Beast.

The God-Tree
In the beginning, humanity was constantly at war with each other. There was no ninjutsu and therefore no ninja, due to the fact that all the mystical energy in the world resided in a divine tree. Once every thousand years, the tree would bear fruit. A woman named Kaguya Otsutsuki consumed this fruit, gaining the powers of a goddess and putting an end to the conflict, albeit through forceful methods. Kaguya later gave birth to a pair of sons who inherited all of her powers. One of these sons would grow up to become the Sage of the Six Paths. Jealous that her sons had inherited powers similar to herself, Kaguya became one with the God-Tree and her body and mind formed the Ten-Tailed Beast.

The Sage of the Six Paths
The legend of the Sage of the Six Paths, the man who became the foundation to the fundamentals of ninjutsu, played a vital role in the series storyline. The Sage's mother finally lost control of her powers and mutated into the Ten-Tailed Beast, wreaking havoc in an attempt to reclaim the power of Kaguya's sons. The combined might of the Sage and his brother was enough to counter the threat of the beast, successfully overpowering it. When the Ten Tails was defeated by the Sage, he sealed the monster within his body. However, as the seal he placed would fade upon his death, the Sage used his power to prevent the Beast's return by dividing its immense chakra into the nine Tailed Beasts with the monster's semi-lifeless body sealed away within the moon where it rotted over time.

Though separate with names of their own, the Tailed Beasts were all linked to each other. But in time, the Tailed Beasts parted ways and became legends in their own right. It was only by the actions of Hashirama Senju, a descendant of the Sage of the Six Paths, that the Tailed Beasts were all placed with captivity of various Ninja villages: Kurama the Nine Tails ended up with the Hidden Leaf Village, Matatabi the Two Tails and Gyuki the Eight Tails with the Kumogakure, Isobu the Three Tails and Saiken the Six Tails with the Village Hidden in Mist before the former escaped into the wild, Son Goku the Four-Tails and Kokuō the Five Tails with Iwakagure, Chomei the Seven Tails in Taigakure, and Shukaku the One-Tail in the Village Hidden in Sand.

Madara's Plot
But Hashirama's rival Madara Uchiha, another descendant of the Sage, took his rival's blood and used it to become a near-genetic match to the Sage. By the time he gained the Rinnegan as an old man, Madara summoned the Ten-Tails's body, the Demonic Statue of the Outer Path back into world to enact his Eye of the Moon Plan to reform the Ten Tails and become it new host to alter reality itself to suit his ends. But as he is near death, Madara conditioned Tobi as his successor to carry out his will through the Akatsuki to capture the Tailed Beasts and then revive Madara through Nagato.

Appearance
The Ten-Tailed Beast's appearance is fairly nebulous for the majority of the story. When Tobi first mentions its existence, the Ten-Tails is a humanoid silhouette with spikes erupting from its back and ten fox-like tails. Kurama's knowledge of the Ten-Tails shows the creature as looking slightly more bestial than what Tobi described, with a more jagged outline and an upper body that looks like a mountain with spiky protrusions. The only constants in all of the Ten-Tailed Beast's appearances is that it has a single eye that resembles both the Sharingan and the Rinnegan, as well as dwarfing the other 9 Tailed Beasts in size.

The Gedo Statue, which is later revealed to be the husk of the Ten-Tails, resembles a withered humanoid with sawed-off branches growing from its upper body, cuffs on its wrists and ankles, a gag in its mouth and 9 eyes. The eyes are initially empty orbs until the statue consumes a Tailed Beast, at which point a pupil appears in one of the eyes. When all nine Tailed Beasts are sealed within the statue, its eyes start weeping blood before undergoing its metamorphosis.

In its larval stage, the Ten-Tailed Beast has a bloated, veined body with a mouth like a cave opening filled with stalactites and stalagmites where its teeth and tongue should be. It has two arms and no legs, so it mainly moves around by dragging itself. The beast has a large spike on its back with smaller spikes growing from the main spike. Its tails are wrapped in several layers of carapace that unfurl as it undergoes its next metamorphosis. When Madara talks about the Ten-Tails, he always envisions it in this form.

In its second stage, the Ten-Tails has a skeletal body with two legs and one arm. Its tails now look like tubes ending in baby hands. The beast's head now has its eye on the left side of its head with its now smaller mouth on the right and an ear on the back. It also has horns on its head and under its eye.

In its third stage, the Ten-Tailed Beast gains a second arm, along with muscles and longer horns. It also possesses the ability to stretch its mouth open the length of its body and regurgitate a large flower that can shoot energy waves. If the beast was allowed to mature further, it would become bloated and grow spikes on the rest of its body, resembling the form it took when the Sage of Six Paths last encountered it. As it was being sealed away by Naruto and Sasuke, the Ten-Tailed Beast reverted to this form once more. Its tails also bore the heads of the nine Tailed Beasts, plus a tenth beast that resembled a rabbit.

After Obito becomes its Jinchuriki, he's able to turn the Ten-Tailed Beast into its Shinju form. As Shinju, the Ten-Tailed Beast transforms into a gigantic flower taller than any mountain with roots that resemble dragons. According to Madara, in the center of the flower is the beast's eye.

When Kaguya reclaims the power of the Ten-Tails and is bombarded with the minds of the nine Tailed-Beasts residing in Naruto, she transforms into something completely different; a Ten-Headed Beast which resembles a gigantic rabbit with a large fleshy mass growing from its back bearing the heads of the nine Tailed Beasts.

Revival
Though the actions of Naruto Uzumaki hampered much of the original plan, Madara was revived through Kabuto Yakushi during the Fourth Great Ninja War while Obito used the trace chakra of Gyuki and Kurama to begin the Gedo Statue's painful regeneration into an incomplete Ten Tails despite Naruto and Killer Bee's attempt to destroy it. Obito and Madara then jump onto the Ten-Tails and interface with the monster as they have it overwhelm the two Jinchuriki and Kakashi. Just as the Ten Tails is about to destroy them, the remainder of the Shinobi Alliance appears to help fight Madara and Obito. Naruto announces that their combined powers will end this war once and for all.

The various ninja all use their signature-jutsu to encase the Ten Tails, while Naruto and several others blind the monster before directing their assault on Obito and Madara. At that moment, the Ten Tails begins to transform into an even more bizarre form. Revealed to be its adult stage, the Ten-Tails begins firing Tailed Beast Balls from its mouth with a recoil from not adjusting to its form, destroying far-away villages and the allied ninja forces' base of operations. By that time, the Ten Tails begins to resist its masters' control before their ties are severed.

Now acting on its own will, the Ten Tails terraformed the area with various storms and earthquakes before gathering strength upon seeing images of the Tailed Beasts and the Sage of Six Paths around Naruto. When the revived Hokage appear and imprison the beast, Naruto, along with Sasuke, Sakura and the remainder of the Ninja Alliance, launch an assault on the Ten Tails. The Ten Tails is able to hold them back by spawning an army of monsters, but Naruto and Sasuke manage to get through and cause it some damage. Just when it seems like the Ten Tails' defeat might be imminent, Obito appears on its head and manages to seal it inside himself, becoming its host.

Obito's Ascent
Despite outclassing every ninja who faces him, Obito is backed into a corner by the combined efforts of Naruto along with the Second and Fourth Hokages. Obito then unleashes the Ten-Tails, which undergoes one more metamorphosis, this time becoming the God-Tree. The God-Tree then sucks dry the chakra of several ninja as it grows to its full size. According to Madara, once the flower on the God-Tree blooms, humanity will be cast into a world of illusion with no end.

After a combining their efforts, Naruto, Sasuke, Kurama and their allies are able to strike Obito, which causes the Ten-Tails to split back into the 7 tailed beasts. Naruto and his allies then extract the beasts from Obito's soul. The God-Tree remains.

Madara's Victory
After Obito's defeat, Madara forces him to resurrect himself. Now fully alive, Madara extracts the Demonic Statue of the Outer Path from Obito and uses it to ensnare all nine Tailed Beasts. They're all dragged in and used to once again bring back the Ten-Tailed Beast. Madara then seals the beast inside himself and becomes the most powerful being on Earth. When Naruto and Sasuke gain the powers of the Sage of Six Paths, they're able to overpower Madara. The God-Tree telepathically tells Madara to absorb it. After this, Madara eventually cornered reunited team 7, the only opponent left after he affected everybody with Infinite Tsukuyomi. However, Black Zetzu stabbed him, much to everyone's shock. Madara was shocked, because Black Zetsu reveals that he is not actually created by Madara's will, but rather Kaguya Otsutsuki's, whom eventually manifests inside him. Madara is eventually consumed by Black Zetsu who manifests as Kaguya Otsutsuki.

Defeat
Eventually, Naruto and Sasuke are able to corner Kaguya and initiate the sealing jutsu that would trap her once again for all time. Her son Hagoromo used his abilities to separate the nine Tailed Beasts from her so that she would revert back to the Ten Tails and the Demonic Statue, unwittingly bringing back Madara. Naruto throws Zetsu into the prison as well to ensure that neither Kaguya nor the Ten-Tailed Beast can ever be brought back.

Trivia

 * 'Shinju' (神樹) literally means 'god tree', and its size and nature have connections to both the world tree and the tree of life, motifs that are present in several real-life religions and mythologies throughout the world.
 * In addition, the story of the Shinju bearing the fruit that was deemed forbidden to consume is very similar to the story of the Tree of Knowledge of Good and Evil (commonly known as the Tree of Knowledge or Tree of Conception) in Abrahamic religions, in which Adam and Eve, the first humans, ate the forbidden fruit despite being told otherwise. Ultimately, while becoming wiser, it lead to their downfall by incurring the wrath of God. For disobeying Him, He forever cursed the two and their descendants. Adam's eldest son, Cain, would later murder his younger brother, Abel, in anger and jealousy following God's rejection of his offering while accepting Abel's, similar to the struggle between the two sons of the Sage of the Six Paths.
 * The wooden dragon-like entities at the base of the Shinju may be a reference to Nidhogg, a dragon or serpent who eternally chews the roots of Yggdrasil in Norse mythology. Likewise with the appearance of the Shinju, it is said that should Nidhogg finally be free of its entrapment underneath Yggdrasil, its appearance into the world beyond its roots are said to herald the coming of Ragnarök, the end of the world and ultimate battle of the gods.
 * When the Ten-Tails was recreated into a tree by Obito Uchiha in the Fourth Shinobi World War, it was initially referred to as the Shinju by Gyūki, but it was later confirmed to be a form of the Ten-Tails.[9] Likewise, the Ten-Tails' tree form was referred to as the Shinju as well in the fourth databook, but it also showed the recreated tree to be another form of the Ten-Tails.
 * When Kaguya was defeated by Naruto and Sasuke, the Shinju, being one with her, was sealed by the Six Paths — Chibaku Tensei within Kaguya's core dimension.