The Celestial Dragons are the decedents of the 20 kings that formed the World Government 800 years ago.
The Celestial Dragons(tenryubito) have ruled over the world for the past 800 years. Said to have the blood of the creators. They are the decedents of the twenty kings that created for World Government. Throughout the passing centuries. The royal familes became increasingly tyrannical and arrogant. Going so far as to wear bubbles over their heads so they wouldn't have to breath the same air as the common people. They are based in the Holy Land of Mariejois, which rest on the Red Line and above the half way point of the Grand Line.
They are not uncommon to have slaves. Either buy purchasing them from the slave market or taking someone off the street that catches their eye. All are to kneel, bow, and make a path for them. To defy their wishes is strictly forbidden. Punishment could mean death for they have the right to kill anyone they wish.
Though the Celestial Dragons seem to possess great influence in the World Government. It's unclear just how much control they actually have. They could more just be figure heads at this point. The real hearts and brains of the World Government are the Five Elder Stars, gorousei. They oversee the Marines, Shichibukai, and the Cipher Pol.