I don't think people really understand this drama because all I'm seeing is complains. Don't watch a drama and start complaining if you aren't going to understand the story that it was telling. I didn't like pt1 but as I began watching pt2 it became obvious to me what the story was all about. I think Right from the start it was obvious to us but we choosed not to see it. First of all, the main theme has always been the seven stars, the king born with the 7 stars, the ice stone and Jonyoen. I will try to break it down if I can.
JBY was never existed, she died when she was in the womb, for that reason JS was reborn through her with the help of the ice stone since the ice stone belonged to her which was why she was able to sense and retrieve it from the lake, it was also why she wanted to destroy it because she knew the effect it will have on humans with greed. She came back to stop the future she saw in her vision.
The seven stars are JU, the person he was when his energy was closed and after it was opened which made him also the king as he was a product of the ice stone as well being that the ice stone is a powerful object. The other stars are all Jang UK friends, PDG, SY, the crown prince, Jin Bo Yoen and her sister JCY. The last star is CY aka Naksu. CY (Naksu) was destined to meet JU being that both fathers were best friends and their children would have met and fall in love with each other which master Lee also to be in conclusion of.
JU was a king because of the alchemy of souls in which he was born, JBY was reborn as a star to help him stop the calamity while Naksu (calling her Naksu because it's easy) was born as a star to be his shadow.
Jonyoen played a theme part in the sense that it's the most powerful place, so powerful that an object like the ice stone and the fire bird could be store there, which was why Jin Mu was obsessed in obtaining it or gaining access to it. Jonyoen collapsing would have been a disaster which we almost saw happened.
People wondering what what happened to mudeok soul or JBY soul since Naksu took over. In Pt1 while naksu was trying to shift soul into another woman but got to enter mudeok body is because JBY made it so, both were stars and both had a role to play in the king (JU) coming into his power. In pt1 Naksu got to live as mudeok/JBY and in pt2 JBY got to live in naksu body. Been that the first priestess JS was never met to be born but she had to be born to see the future come to pass and stop it that was why she was able to leave at the end and gave way to the light and shadow which was Naksu to live because she never really got to live.
I summarised it well in my head than I did writing it out, but all in all I really understand the drama and loved it. I guess the problem is the confusion of it all, in my own opinion it shouldn't have been divided, while watching it I refused to think of it as pt1 and pt2 and just sat it as 3YearsLater in my head which made me understand it better and got the bigger picture. I honestly loved the 3 years later more than the first part.