We know very little about Brook’s past, compared to the other Straw Hats and various characters in the series; he was a member of the Rumbar Pirates who traveled with Laboon, explaining his motivation in joining the SHP, but he briefly mentioned being the leader of a battle convoy of a “certain kingdom.” Now, I’m in no way trying to connect that to the Ancient Kingdom, but I believe the skeleton knows more than he’s letting on.
Brook was 38 when he died, which was 52 years before the current storyline. He was stuck in the Florian Triangle for the first five decades of that time, but he had still lived quite a while before dying (the first time). Although the Marines suppressed all information about the crew and their captain 40 years ago, there’s a good chance that the Rocks Pirates could have been active for a while before their defeat/disbandment, and if so, their notoriety must have spread to at least other seafarers, if not the entire world. Since Brook was active even before joining a pirate crew, and by all indications a soldier somewhere who was capable of ably defending a kingdom, there’s a good chance he may have even clashed with Rocks et al.
If you really wanna dive down the rabbit hole, you could speculate that Brook’s kingdom and original allegiance lay somewhere important, such as Mariejois or...God Valley...but I’m not prepared to go that far just yet. My point is just that he must have heard about the most outrageous and notorious pirate crew of his day, and I hope he spills the beans if this ends up being true. Thoughts?
Brook was 38 when he died, which was 52 years before the current storyline. He was stuck in the Florian Triangle for the first five decades of that time, but he had still lived quite a while before dying (the first time). Although the Marines suppressed all information about the crew and their captain 40 years ago, there’s a good chance that the Rocks Pirates could have been active for a while before their defeat/disbandment, and if so, their notoriety must have spread to at least other seafarers, if not the entire world. Since Brook was active even before joining a pirate crew, and by all indications a soldier somewhere who was capable of ably defending a kingdom, there’s a good chance he may have even clashed with Rocks et al.
If you really wanna dive down the rabbit hole, you could speculate that Brook’s kingdom and original allegiance lay somewhere important, such as Mariejois or...God Valley...but I’m not prepared to go that far just yet. My point is just that he must have heard about the most outrageous and notorious pirate crew of his day, and I hope he spills the beans if this ends up being true. Thoughts?

