I think Xuanji believes the deputy palace lord’s words and unsealed Liuli cup, to save Yu Sifeng she releases the heart and soul of Luo hou Jidu, and let him save Yu Sifeng. In the trailer, Xuanji actually released Luo Hou Jidu and saved Yu Sifeng, but the price was that Luo Hou Jidu entered Chu Xuanji's body.
And Xuanji asked Yu Sifeng not to forget her before losing control of the body. Of course, Yu Sifeng would not forget her. In order to awake Xuanji's consciousness, he was willing to be the demon slave of Luo hou Jidu. It's just that a good wife has become such a "boyfriend" as Luo Hou Jidu, Yu Sifeng doesn't know if she will respond. Of course, Sifeng's efforts are not ineffective. Xuanji can occasionally take charge of her body and become a woman's appearance, but most of the time Luo Houji still appears.
I think when Yu Sifeng was injured badly during the war may be Xuanji takes back the control of her body weird theory right?