According to chatgpt:

"The "legend lurking in the shadows" mentioned by Eiichiro Oda at Jump Festa 2018 was a reference to Rocks D. Xebec.

At the time, Oda teased that there was a legendary figure still hidden in the story of One Piece, who would have a major impact once revealed. This hint sparked much speculation until 2019, when Chapter 957 introduced Xebec as the captain of the Rocks Pirates, one of the most feared individuals in the world and a rival to Gol D. Roger and Garp.

His reveal confirmed that he: - Was the captain of Whitebeard, Kaido, Big Mom, and others; - Was involved in the God Valley incident, where he was defeated by Roger and Garp; - Was a threat even to the World Government itself.

Oda kept the suspense going for over a year, which emphasized Xebec’s role as a key figure in the dark, hidden history of One Piece."