Nine-Tailed Demon Fox's Attack

Twelve years before the start of the series, after the end of the Third Great Shinobi War, the Nine-Tailed Demon Fox attacked the village of Konohagakure. Nobody in the village was able to stop the fox until Minato Namikaze, the Hokage at the time, arrived to seal the fox into his newborn son, Naruto Uzumaki. The village suffered heavy casualties during the attack, such that few of the villagers would forgive Naruto (who they viewed as the fox) for its actions.

Though the demon fox had attacked many settlements regularly throughout history for no apparent reason, Jiraiya and Itachi Uchiha speculate that Madara Uchiha was behind the attack on Konoha. Madara denies this. In the anime, the fox is revealed to have left behind large amounts of its chakra during the attack that, when gathered by Kazuma, allowed him to turn Sora into a Pseudo-Jinchūriki.