Avatar: the Four Elemental Masters

This world is dominated by the four elements (water, earth, fire, air) and produces four tribes, but to be precise, there should be two kingdoms and two tribes, the water tribe, the air tribe, the earth wall kingdom and the fire kingdom. There is a person named Avatar, who can control these four elemental forces at the same time and has the ability to save the world. But the supernatural power had disappeared for almost a hundred years, and Lu Xiubai woke up at this time. His mission is to find the Avatar to save the world and his childhood goddess, Azula. Note: The only CP in this book is Azula, who won't have a harem or anything, but will try hard to let everyone live in Korra's era. An will probably die, because Korra will not be reincarnated until he dies.