Mount Siming was one of the Eighteen Rebel Kings.
It was ruled by the Three Great Kings.
The reason Xu Xing tricked the Heavenly Kun King into becoming sworn brothers with him wasn't because he enjoyed making such alliances. Rather, it was because he had seen through the power structure within Mount Siming at a glance. If I don't become sworn brothers with the Three Great Kings, then my best outcome would be to become someone like Zhang Liang or Xiao He.
