There is no exact date on this strory.
It says that in ancient times a Mireau, a sky dragon, had nine sons. These sons were to stay in their heavenly abode until they could grow into sky dragons themselves. Then they could serve as benevolent rulers of the weather.
One of the sky dragons’ sons, however, could not wait to rule. He decided to come to earth and begin his own empire. He slithered from his heavenly abode and fell to earth as a imoogi, a giant serpent. There he began to raise his army. He gave special powers and privileges to the high ranking members of his army. They could magically dissapear in a mystical blue light. They had magic wands that inflicted death from a distance. They were given large harems in which to satisfy their bodily pleasures.
When the army took over a kingdom, the men of the kingdom had two choices: join the army or die. If they joined, they were castrated and their wives and daughters were given to the high ranking members to fill their harems. The men who joined became fanatical followers. The army seemed unstoppable.
The imoogi’s empire grew great, and he set up a great palace in the mountains far to the West. In those mountains, he built a mystical pathway to the stars. There, he grew into a mighty Gyo, a mountain dragon. From the stars, the Gyo summoned terrible demons to whom he gave the most beautiful women to produce powerful magicians as offspring.
Although many warriors went up against them, they were demolished by the Gyo’s army. But some small villiages in Korea, villiage which had no true warriors, none-the-less housed groups of Sulsa. The Sulsa could sneak out in the nighttime and poison the supplies of incoming armies, sabotage their weapons or even kill their leaders in their sleep. This kept the army away for longer than in many other villiages.
Still, the Sulsa knew that the armies could not be held off forever. More and more villiages were falling to the army. So, one of them decided to see if he could destroy the Gyo in his lair. He travelled west for many days to the palace of the Gyo. Once there, he found a way to ascend the magical path to the stars. Here he called out to the Mireu, the father of the Gyo, to look on the distruction his son had wraught.
When the Mireu saw what had happenned he was very displeased with his son. He gave the Sulsa mystical boats which could ride in the skies and in the stars. From these boats, they could fight the demons that the Gyo had summoned. He gave them holy water to drink, so that those who had become fanatical followers of the Gyo would have their minds healed and their bodies would be restored to full manhood again. Once healed, the former followers joined with the Sulsa. The great army began to fall person by person. Wives were returned to husbands.
Eventually, the army the Gyo himself had raised turned against him. The Gyo appealed to his father, the Mireu for forgiveness. The Mireu had mercy on his son and took him back to the heavenly kingdom, leaving the earth, once again, at peace.