Du Youqian and Xia Chuchu went way back. Ever since he became the Celestial Master in this life, he had dealt with her on several occasions.
Later, during the battle where Pang Master ambushed the Core Formation Masters of the Nine Great Sects outside a Secret Realm, Xia Chuchu had ostensibly been gravely injured and fled. In reality, she had secretly pledged her allegiance to Pang Master. She did it to obtain the resources needed for further advancement and for the chance to follow him to the outside world.
However, during the battle at Soul-shaking Mountain, Xia Chuchu was sternly warned by Nong Jiasong, who was the top expert in the Demon Sect Camp at the time. She promptly corrected her ways, so she was spared any trouble during the subsequent reckoning.
Nevertheless, she still had to keep a low profile for a century or two, waiting for the matter to blow over.
