Nine-Tailed Demon Fox's Attack

The attack of the Nine-Tailed Demon Fox took place after the end of the Third Great Shinobi War, not long after the birth of Kushina Uzumaki and Minato Namikaze's son, Naruto Uzumaki. Possibly, Madara Uchiha summoned and controlled it using Mangekyo Sharingan and ordered it to attack Konohagakure, although he claims it was a natural disaster.

Massacre
During the attack, the demon fox was destroying trees and several places. Iruka Umino's parents were among those who were bravely fighting the demon leaving Iruka to see them. But he was taken for protection by two unknown Konoha ninjas. Several shinobi were trying their best to destroy the fox. Presumably, Kushina could have been killed at this time.

Legacy of the Fourth Hokage
Konoha's population started to decrease. Until one ninja, Minato Namikaze, the Fourth Hokage, with the cost of his life used the Dead Demon Consuming Seal and the Eight Trigrams Seal Technique to seal the demon into his newborn son, Naruto Uzumaki making him the host of the Nine-Tailed Demon Fox.

Trivia

 * Contrary to popular belief, this did not happen on Naruto's birth, October 10, as the demon fax was sealed after Naruto's umbilical cord fell off. Thus, is happened about a week after Naruto's birth.