When One Piece initially began the Fishmen Island arc, it seemed like just a way to introduce the New World after the major timeskip, and didn't seem like it would have major consequences for the series down the line. But towards the end of the arc, it was revealed that there was a greater mystery about the mysterious "Poseidon" weapon that could command the Neptunians of the sea and cause a ton of damage. This was the result an even more mysterious prophecy carved into the Poneglyphs, and as the Wano Country arc continues to follow Gol D. Roger, we've finally seen the root of where it began.
Although it was eventually revealed that Shirahoshi the Mermaid Princess was in fact this Poseidon weapon, and that she was the prophecy, the latest chapter of the series explains this prophecy in full as Gol D. Roger inquires about it to the young Sharley, who at the time of this current flashback was three years old.
Chapter 967 continues Kozuki Oden's accounts of his travels with Gol D. Roger's crew, and they head to Fishmen Island. It's here they meet Neptune, who is currently a knight at the time of this flashback. Neptune tells Roger that he's worried about a prophecy that will destroy their walls (that doesn't come into play for real until years later), and Sharley explains it in full.
She explains that the Neptunians are restless because they are waiting for the mermaid princess to be born, and Neptune is shocked by this as he has yet to take the throne officially. His father passed only a few months ago, so he wonders how this prophecy ties into the one about a mermaid who can talk to the Neptunians will be born. Sharley confirms that this mermaid will be born in ten years time, but while she will be the weapon Poseidon, Roger's more interested in those that tried to weaponize the future Shirahoshi in the first place.
Roger's crew eventually moves on with their journey, but fans know that this prophecy indeed comes to pass years later. This puts a whole new emphasis on the Fishmen Island arc, and makes it into much more than just a narrative buffer into how the New World operates.
Eiichiro Oda's One Piece first began serialization in Shueisha's Weekly Shonen Jump in 1997. It has since been collected into over 80 volumes, and has been a critical and commercial success worldwide with many of the volumes breaking printing records in Japan. The manga has even set a Guinness World Record for the most copies published for the same comic book by a single author, and is the best-selling manga series worldwide with over 430 million copies sold. The series still ranked number one in manga sales in 2018, which surprised fans of major new entries.