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Since Oden was very confident in stating that there would be a giant war in 20 years and the protagonist(s) will rise to participate in this war, I guess he truly believed it.
So the question is, where did he learn about this? I see three potential sources:
So the question is, where did he learn about this? I see three potential sources:
- Shyarly: This one seems the most likely as she can predict things to specific dates. However, it would be weird that she prophesied all of that and yet, she prophesied Luffy would destroy Fishmen Island. Shouldn't she be able to recognize the protagonist(s)?
- Sea-kings: It was shown that Oden could hear the voice of all things and he heard some sea-kings discuss about the rise of their King (and another king). But the problem is that the Sea-kings weren't specific with dates, they estimated 10 years to birth and another 15 to grow. They didn't even give an estimate for the other King. So, how did Oden become so confident in the '20 years' thing that the scabbards were sent exactly 20 years into the future?
- Laugh Tale: Laugh Tale is stated to contain the history of the past written in stone. But does it also contain future events written in stone? I doubt it. Because then, the implications are huge. It would mean that everything in the OP world is following a predetermined set of events and people's will has no bearing on anything.