The Nine-Tailed Demon Fox (九尾の妖狐, Kyūbi no Yōkō?, English TV "Nine-Tailed Fox Spirit/Demon") 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. Twelve years before the start of the series Madara Uchiha used his Mangekyo Sharingan to have the fox attack Konohagakure. 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. 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. Minato left a key to Jiraiya that could either strengthen or weaken the fox's 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.
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