Dragon Slayer

There are seven dragon kings in the world, who originally lived in harmony with all races, but after discovering that they could enslave humans, they became more and more cruel. Someone proposed to the dragon kings that human beings should be given freedom, and the dragon kings laughed that this person was looking for fun for themselves. So that man created his own dragon-slaying technique, defeated and sealed the dragon kings, and predicted that a thousand years later the dragon kings would break through the seal and seek revenge on mankind, so he ordered future generations to train dragon-slaying warriors who can fight the dragon kings in every generation...