Sites like Vecteezy or Freepik offer free skull-and-crossbones icons and illustrations for personal or commercial use. Which of these are you most interested in exploring?
Sites like Project Gutenberg or Internet Archive offer free downloads of historical texts regarding 18th-century piracy (the "Golden Age") and academic papers on secret societies. Skull and Bones Free Download
The game is included in Ubisoft+ , a monthly subscription service that allows you to play their entire library. Skull and Bones Free Download