Xebec, commonly known simply as Rocks, was the captain of the Rocks Pirates.
He was prominent 40 years ago, before the Pirate King Gol D. Roger came into power.
Based on stories, Rocks appeared to be an incredibly vicious and ambitious man, having no care as to who he attacked as long as he could garner power for himself and leading his crew to destroy whomever interfered with his ambitions.
Although Rocks' individual strength and abilities are unknown but he was an extremely powerful and charismatic individual and possibly the strongest person of his time.
Gol D.Roger is unarguably one of the strongest characters in the series. 38 years ago, he teamed up with a certain Marine to defeat Rocks.
Rocks is seen wielding a sword during the fight with Roger and Garp in God Valley.
Many of Rocks' subordinates would go on to become extremely notable pirates, particularly Edward Newgate, Charlotte Linlin, and Kaidou, who became three of the Four Emperors ruling over the New World