When Kangxi, emperor of Qing dynasty went to hunt, he met with beautiful girl, youngest daughter of the dragon.
Kangxi asked her from king of dragon to be his woman.
The king of dragon would agree unless Kangxi promised him, first, don't force and second protect. Kangxi gave his promise. The king of dragon warned if he could not keep his promise, he would destroy him and his country.
Kangxi took dragon daughter to palace and made her be his private eunuch. No one knew, only his imperial eunuch.
The emperor and her developed until she fell in love with the emperor.
Then one of Kangxis' consorts found out he hid girl inside his room. She took chance when he was away, and took life of the girl.
She did not realize her origin.
Her death brought anger to dragon world and the king of dragon flooded the empire.
In order to save people and country, Kangxi needed to go to fight alone with the dragon.