For hundreds of years, the Fire Kingdom is known to be the most powerful among all of the Elemental Realm. They house and protect the Elementium, the most powerful gemstone that controls nature's four elements.
The Fire Lord is known to be the mightiest ruler. However, it is no secret the lengths he have gone through in order to keep his power. He gets rid of anyone that dares usurp the Fire Kingdom's supremacy, committing sins and treacherous acts that resulted in multiple, albeit failed, rebellions.
When the Fire Lord takes the prince of the Valley of the Winds, Vale, as his adopted son and prisoner, the Wind Sultan then arranges a coup, gathering support and forming alliances to end the Fire Lord's wicked regime.