Before I state my theory, I want to let you guys know that everything up until episode 930 in the anime is up for discussion, anything in the Manga that extends beyond the events of episode 930 are not allowed and will be reported.
Okay, so you know in episode 194 – Sky Island. Robin finds the Poneglyph on the Golden Bell, and deciphers it and finds nothing of what she truly wants (The void century). However, there’s a small writing on the right of it with Roger’s message stating “I will lead this text to the ends of this earth” Aka – Raftel. She calls this “Rio Poneglyph” which she describes it as reading all of the Poneglyphs as one message rather than separate; hence discovering what the Void Century is.
Another thing, Road Poneglyphs are the only things that aren’t there as they are a guide to that island; leaving only “Historical and Informational” Poneglyphs scattered. But Roger clearly knew how to read them or had someone that knew, so they could have written it down as a book and took it with them in their journey.
Please, share your thoughts without any spoilers, I would love to hear thoughts.
Theory by Marvione