Still in the past, Su Yi Shui is training under Mu Qing Ge. He was chosen as a disciple for two main reasons: on one hand, he has 5 spiritual roots as Mu Qing Ge, hence he is a worthy successor, and on the other hand, by dissolving his killing intent, she wants to save the world from a potential disaster. Mu Qing Ge proves to be a very insightful and intelligent master. (Source: DanielaZ at kisskh)