One Piece is filled with terrifying and powerful crews, from the Big Mom Pirates to the Straw Hats. However, one of the most feared and legendary crews was the Rocks Pirates. Among the crew were some familiar faces: Big Mom, Kaido and Whitebeard. These three would go on to lead their own crews after the Rocks Pirates disbanded and later become three of the Four Great Emperors. But, if they were so great, why did they disappear?
The answer lies 40 years in the past, before the time of Gol D. Roger. The captain of the Rocks Pirates was a man named Rocks D. Xebec. He was ruthless, cutting down anyone that would get the way of his dream of becoming King of the World. He would target places and people that would cause the most destruction and anarchy, not hesitating to take out World Nobles. Not only this, but he was interested in forbidden knowledge that the World Government wanted to be forgotten.
His crew were equally as ruthless and held each other with little respect or regard. 40 years before the current storyline, a group of pirates came together to form one big crew on the island of Hachinosu, led by Xebec. They were all-powerful and had big personalities, so they would often clash with each other. For Xebec to bring them together as a crew was a feat in itself. It shows just how charismatic he really was. Considering that three of his mates would go on to lead their own formidable crews, they must have learned a lot under their captain.
Unfortunately, there was a conspiracy afoot. Gol D. Roger was vying for the title of the Pirate King, and Xebec stood in the way of that. Xebec was considered to be Roger's biggest rival at the time, and some would consider him bigger than Whitebeard. So in order for Roger to go on to create his legacy, Xebec needed to be taken out. So, he and his crew teamed up with the Marines, led by Monkey D. Garp.
While the Rocks Pirates were traveling through God Valley, the Marines and Roger's crew attacked them. It was this union that finally brought Xebec's reign of terror to an end. It would also be this victory that earned Garp's nickname as the Hero of the Marines.
Because Xebec knew so many of the World Government's secrets and taboo subjects, they decided that he needed to be forgotten. All records of his existence have been erased. The only ones who remember him were alive during his time at sea, though his former crewmates don't seem to talk about him all that much.
Because they clashed with each other so much, it's natural that they wouldn't continue journeying together -- until now. Big Mom and Kaido have once again come together to try and take over the world, taking after their former captain. They are basically keeping his legacy alive, even though they are only in it together to reap the benefits.