The Shinju is the embodiment of the Divine Tree that lashed out at humanity for kaguya eating its forbidden fruit and taking its power.
It was long ago before the founding of all the hidden villages, during an era of endless wars between mankind, the God Tree only as a mere fruit tree and everyone at the time had revered it as a god and to end this endless war between humans a princess by the name of Kaguya Ohtsutsuki who traveled from a unknown place was assigned to protect and worship the Ten Tails and its forbidden fruit since it's fruit only grew every 1,000 years. One day she made a unforgiving choice to eat from it while to protect her and her tribe and then from that day on was gained with the Shinju powerful divine power's and with this she used to end all wars during her era this was only after she had given birth to two sons and one of them who in the later years was later to written in a tablet to be created by Hagoromo Ohtsutsuki and he to was also given the power of Chakra and the Rinnegan and stated to be the first born with chakra other then his brother. One day the Ten Tails itself had taken a monstrous form so enrage and had it took upon it's own human and animal like form as the beast watched with it's eye at the world and seen how it looked at the violence the monstrous being was confused about humankind and in anger started to attack the world it created killing Kaguya and only to be later stopped by her two son who imprisoned the beast inside himself. This thus made him the worlds very first jinchuuriki. Hoping to wipe there mothers sins off the world by sealing it inside there very bodies being embodied with its sinful body marks.
The Shinju is far different then its tailed very own beast counterparts it's appearance is shown to be both an animal and humanoid form and has only one giant eye that has three layers of three-tomoe Sharingan in it but a Rinnegan style form.
The shell also known as the "Gedo Mazou" is made out of nearly indestructible wood from Madara Uchiha's wood release he had to make a perfect vessel to contain the Shinju raw pure chakra. It appears to have chains around its wrists and ankles and has 9 eyes of variant sizes.
In the time of endless time, the Ten Tails was a monstrous rampaging beast, referred to as the Shinju. To save the world Hagoromo performed a jutsu that would seal it inside his own body and his brothers body, thus making them the first ever Jinchuuriki's. The Shinju would eventually be free once either of them died so Hagoromo had separated the Shinju's chakra from its body. The chakra would be split into 9 separate tailed beasts which would become the 9 bijuu that are known throughout the Naruto universe in the present day. He then performed another jutsu that would incase the body in a sphere of earth and sent it into the sky, where it would become the Naruto universe's own moon.
After its Chakra was separated, the 9 bijuu were scattered across the world, over time the first eight were brought under the control of the 1st Hokage, who in the spirit of keeping tensions low between the five main ninja-villages, allocated bijuu to each village. They in turn would use their own versions of the original soul sealing jutsus to create their Jinchuurikis. Madara Uchiha would then take control of the Kyuubi and fought the 1st Hokage, and Hashirama Senju won and sealed it inside his wife Mito Uzumaki.
Tobi's plan, in which he has had Akatsuki capturing Jinchuuriki's and extracting their bijuu for years, will become complete with the capture of the Hachibi (Killer Bee) and Kyuubi ( Naruto Uzumaki). Once he has all nine, he will be able to restore the Juubi and become the Juubi-Jinchuuriki himself, increasing his already omni-powerful Eternal Mangekyou Sharingan. This will allow him to create the most powerful genjustu, "Infinite Tsukuyomi" (Mugen Tsukuyomi), which he will place on the moon itself. The jutsu will reflect on the entire planet, making everyone one with himself.
To stop this, the five great ninja villages and the Iron Country, headed by the current Raikage known as Ay, have formed the first ever Shinobi Alliance to participate in the fourth great ninja war, specifically for the purpose of fighting over Killer bee and Naruto against the remaining members of Akatsuki and the seven bijuu Madara has under his command.
The first true body appearance of the Gedo Mazou of the Shinju shell was shown in battle against a already beaten army of shinobi allies rest until Tobi kumai himself there and when they notice he had arrived in the distance he already prepared to summon the Geod Mazou to the field. Suddenly They all prepare for the sneak attack planned by Tobi causing both Choji and his father to face the monster which they failed at because of it's unknown power. So the captain of the ally force prepares a mountain justu hoping to finally stop and end the beast and as the ground shifted up the Gedo showed its true steength by holding both sides with its bare hands and it slowly walked toward the army as they are all infear of the power of the creature had shown them. As tobi kamui himself again to a certain position to take the sage's tool from them containing Kinkaku & his brother Ginkaku which both have kurama chakra inside he was stopped by the hihgtly intelligent Shikamaru which cased him to order the gedo to stop the shadow possession by running at unknown speeds qiuck enough to jump from one side to the coast nearly killing shikamaru. They are later shown to be in a open space while tobi is sitting on the gedo mazou waiting and planning his next move until he ran into naruto where they had an all out "tailed beast" battle which caused him to chain up and absorb the tailed beast's into the gedo mouth storing them like batteries. The Shinju true form was shown later as Madara single handly defeated the kages in battle as they re-joined they dded the chakra to the gedo which made it go insane for a short period only to be thought destroyed by Kurama and Gyuki but tanked the giant tailed beast ball then the battle finally started as the Shinju with raw strength hit both of them along with the team of heroes with a single blow. As Kurama explained to naruto before trying to communicate with it that the Ten-Tails is a mindless beast that doesn't have emotions.