Nine-Tailed Demon Fox

All those who have tailed beasts inside of them have to fight it off for control of their body, such as Gaara, Kabuto, and certainly Naruto.

Whenever Naruto is in real danger, the fox chakra activates, giving him extraordinary strength, though he is much less
humane and much more demonic and animalistic. Now, Naruto is forced to use more and more of the fox's chakra to protect himself from dying and to protect others, but the battle truly begins when he starts getting the first tail, and then the second, third and fouth ones, with the fourth being the newest one in the manga.

The first three tails are very powerful, but Naruto still manages to contain the spirit of the fox inside of him, using only the fox's red chakra, and still maintaining control in the battle. However, when he goes with the 4th tail, he about kills Jaraiya, as Naruto almost completely loses it, and that is not all.

With the tails, comes a serious drawback. Every time he sprouts a tail, he starts bleeding from the pores in his skin, though the fox repairs the skin, but this constant repair wears down and wittles down Naruto's lifespan.

In the end, Naruto cannot really die by the hands of anyone but his own fox demon, as the demon shortens his lifespan more and more with more and more tails. Therefore, by fighting tougher and tougher battles, he will sprout more and more tails, especially if he finds out that his sensei/grandfatheresque sensei croaked (pun intended) fighting pein. When he fights pein, he will no doubt have to go with all four tails, and undoubtedly, have to sprout at least one or two more tails, if not 3 or 4 more tails.
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