Avatar: Last Airbender: Revenge of the Phoenix King

They thought the war was over with the defeat of the Fire Lord. However the powers that be, looking from outside decided that their struggle was only just beginning. Former Fire Lord Ozai, now made into a demon of pure fire, seeks not to rule a world that is reborn from the ashes but to burn it down to spite those who thwarted him, starting with the Avatar and his son.