The Nine-Tailed Demon Fox (九尾の妖狐 Kyūbi no Yōkō?, English TV "The Nine-Tailed Fox Spirit") is the most powerful of the tailed beasts. Its power is such that it is regarded as an age-old natural disaster, as a single swipe of one of its nine tails is enough to raise tsunamis and flatten mountains. The demon fox's attacks were seemingly random, the only apparent reason for its appearances being its attraction to places of human malice. The nine tails of the demon fox is a reference to the belief that a fox may possess as many as nine tails. Generally, an older and more powerful fox will possess a greater number of tails, and some sources say that a fox will only grow additional tails after they have lived for a thousand years. After that period of time, the number increases based on age and wisdom.
The demon fox attacked Konohagakure twelve years before the start of the series. None of the ninja of the village were able to defeat it until the Fourth Hokage, Minato Namikaze, arrived to deal with it. Minato sacrificed his life to seal the fox into the body of his newborn son, Naruto Uzumaki, with the Dead Demon Consuming Seal. Rather than completely seal off the fox's great power, however, Minato only sealed away a portion of it, giving Naruto access to the rest for unknown reasons. Prior to performing the seal Minato gave a key to Jiraiya to either strengthen or weaken the seal should the need arise. The seal naturally becomes weaker as the years pass, and in the event that it is broken the demon fox will be released.
Jiraiya suspects that the fox's attack on the village was not a random occurrence and that it was instead summoned by Madara Uchiha. Although the idea that the fox could be summoned by a single person is regarded as impossible, the fox seems to know Madara; during its encounter with Sasuke Uchiha in Part II, it states that Sasuke's chakra, like Madara's, is more sinister than its own. It also seems to be familiar with the Uchiha clan, though its relationship with the family is unclear.
The only initial indication that Naruto is the fox's host are the whiskers present on his face and the seal on his stomach. He is also able to heal from injuries and fatigue in very short periods of time, though he never finds this as odd. It is not until he learns of the fox within him that Naruto begins to make more use of it. On numerous occasions throughout the series Naruto calls upon the fox's immense chakra supplies in battle to allow him to perform jutsu that would be impossible for ninja of his age and rank. Naruto can access its chakra in two ways: he can succumb to rage, allowing the fox's personality to influence his own and making him more bloodthirsty. Alternatively, he can enter his own subconscious and converse with the fox to ask for a donation of its strength. In these instances the fox is shown to have a loathing respect for both Naruto and Minato. While the fox initially complies to Naruto's requests out of curiosity, it later does so in an attempt to further its influence over Naruto. It also acts in order to preserve its own well-being, as Naruto's death would subsequently result in its own.
As Naruto draws upon additional amounts of the fox's chakra, the fox begins to manifest itself in the form of tails. Naruto can only currently maintain control over his mind for the first three tails; at the fourth, he loses all sense of self and readily attacks anyone nearby. In these instances, the fox can only be stopped by suppressing its influence, either with Yamato's Mokuton abilities or with a special seal created by Jiraiya that is directly applied to Naruto's body.
Akatsuki originally assigns Itachi Uchiha to capture the demon fox, but this task is later given to the organization leader, Pein.
