In the end, Gu Lingyue lost her way and accidentally saved an unconscious old woman. The old woman was skilled at using Gu Worms to poison people. In order to repay Gu Lingyue, she proposed to take her as a disciple and told Gu Lingyue that once she became her disciple, she would be loyal to her Lord together with her.
As long as she could kill Gu Qingxue, Gu Lingyue was willing to pay any price. Thus, she agreed to the old woman's request to start with Prince Heng. She planned to use Prince Heng to get rid of Gu Qingxue's biggest backer, Rong Zhan.
Furthermore, Gu Lingyue and the old woman's owner seemed to have a grudge against Rong Zhan and had always wanted to get rid of him.
