When gu yangcheon goes to the black palace he is also accompanied by namgung biah. His father is also looking for gu heebi. Anyways when he goes to the palace he first rescues gu heebi who was imprisoned with who if I remember correctly was the hao clan lord. In the palace yangcheon sees a child who looks like wi seol ah. But the child is the heavenly demon. Another spoiler far in the story the heavenly demon and wi seol ah are sisters. I could go on and on more with the story but just go to novelbin and read it. It's really fuc**** good