Twelve years before the start of the series Madara Uchiha used his Jiraiya that could either strengthen or weaken the fox's seal should the need arise.
Twelve years before the start of the series Madara Uchiha used his Mangekyo Sharingan to have the fox attack Konohagakure. Minato left a key to Jiraiya that could either strengthen or weaken the fox's seal should the need arise.