Wow... just saw the spoiler. I've always thought that Nagato was incapacitated in some way or another for using the six bodies of Pein to fight rather than fighting himself. What brought him into this condition would be made clear when Nagato tells it himself, or maybe Konan. Maybe he had an epic battle, probably with Hanzou as somebody said, where his body reached it's limits and placing him in this condition. The story did go that he did fought Hanzou on his own. Yes, he did defeat Hanzou but because of Hanzou's ability, strength and skills, Hanzou could have left Nagato this way. Then, only with the power of Rinnegan, he could have found out that he can control up to 6 bodies to do his actual fighting for him, while he is in this state. Or maybe, he's not as incapacitated as he looks. These are just my thoughts though

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