Douluo continent was born in response to robbery

The earth, a classroom, a black dress, a gray dress, a man and a woman. The man held a fairy sword and chose three teenagers, erasing everything about them on the earth. Three teenagers were forced to cross the Douluo continental plane and face an unknown disaster. Three six-year-old boys felt the information in their brains. Nine swords were robbed and burned, recalling the words of a man and a woman: "there will be a disaster on Douluo continent." However, what disaster needs nine swords and burning to deal with?