The Nine-Tailed Demon Fox is the most formidable of all the tailed beasts and is known as the "King of the Tailed Beasts." Its immense power can cause natural disasters; a single swipe from one of its nine tails can generate tsunamis and flatten mountains. One of its unique abilities is access to nearly limitless chakra.
The demon fox's attacks appeared random, seemingly drawn to places filled with human malice. It attacked Konohagakure twelve years before the series began. None of the village's ninja could defeat it until the Fourth Hokage, Minato Namikaze, arrived. Minato sacrificed his life to seal the fox inside his newborn son, Naruto Uzumaki, using the Dead Demon Consuming Seal. However, instead of sealing away all of the fox's power, Minato only sealed a portion of it, leaving the rest accessible to Naruto for reasons unknown. Before performing the seal, Minato gave a key to Jiraiya to either strengthen or weaken the seal if necessary.
The seal naturally weakens over time, and if it breaks, the demon fox will be released. However, due to the bond between Naruto and Kyuubi, if the seal breaks, Naruto will die immediately. Similarly, if Naruto dies while the seal remains intact, Kyuubi will also perish. Therefore, Kyuubi uses some of its chakra to enhance Naruto's healing abilities and sometimes "takes over" by lending Naruto its chakra without him asking, to keep him alive.
