As we know, Rocks Pirates were a legendary pirate crew that were the most powerful during their era, 38 years before the current storyline.
They were led by Rocks D. Xebec. Kaido, Big Mom and Whitebeard were members of this crew, along with many other pirates who made a name for themselves such as Shiki “The Golden Lion”, “Captain John”, “Silver Axe” and “Ochoku”.
The crew strength at the time made them the most threatening pirates to the World Government.
Inside Marine HQ, Sengoku even says that Rocks D. Xebec’s ambition was involved with many of the world’s taboo subjects. This likely involves the Will of D., but what other subjects did he know about? The Void Century? The Road Poneglyphs? How much did his influence lead him to know about the world’s dark secrets?
38 years ago, the Rocks Pirates attacked several World Nobles and their slaves on a island known as God Valley.
On that day, Monkey D. Garp defeated the crew, earning him the title of “Hero of the Marines”.
The Marines listening to Sengoku talk about how they never knew which of Garp’s heroic feats gave him that title. Sengoku adds that Garp tends to keep mum about the whole ordeal aside from the known historical facts.
The first reason Garp likes to keep quiet about the battle is one that never made it into the public record. Garp had to fight alongside a pirate.
The other reason is that Garp fought to protect the Celestial Dragons. The introduction of Celestial Dragons into the story and Garp’s secrecy about protecting them surprise and confuse the present Marines. Sengoku explains that Garp’s moral code does not allow him to protect Celestial Dragons. It’s also the reason he never accepted the rank of Admiral, because he would be directly under their control.
This is particulary interesting, since every single D. in the series has gone against the Celestial Dragons, being the natural enemies of the Gods: Garp refuses to serve them, Dragon wants to overthrow them, Luffy despises them and punched one of them, Roger was the Pirate King and great threat for the Celestial Dragons. The same threat is now represented by Blackbeard. Rocks led his crew on a mission that endangered the World Nobles and their slaves. Rouge bore the child of the Pirate King, Saul opposed the decision of Celestial Dragons to destroy Ohara and Law went directly against Doflamingo.
The true story of the event known as the “God Valley Incident” is that Garp and Gol D. Roger met there by chance, teamed up, and defeated the Rocks Pirates.
This revelation surprises the Marines most of all. Sengoku goes on to say that God Valley is no longer found on any maps. In reality, the entire island disappeared without explanation as if it never existed. Sengoku rephrased his description, saying that an island the World Government wanted to keep secret disappeared.
How much relation this island has to Raftel? Raftel being an island of devils and God Valley being an island of gods seems to create an intersting parallel.