I've seen a quite plausible theory emerging after episode 18. Here it is for more visibility:
Songrim's founder after destroying almost all of the ice stones kept one to save his dying love - powerful blind priestess and Jin clan's founder. He either sealed her soul inside the ice stone, then gave it for safe keeping to Jin clan. Or he used the ice stone to revive her and after she eventually died of old age with the ice stone inside, her soul got absorbed into it. Mama Jin's unborn baby (because it was already dead and had no soul?) was saved by Jang Kang with that same ice stone and priestess' soul transmigrated to the baby. So Bu-Yeon's soul is Jin clan founder's soul, that's also why she's blind. There had to be some connection to the ice stone and her immense powers - there it is.
-The way Bu-Yeon spoke to Naksu in episode 18 kinda felt like she had wisdom beyond her years.
-Talking artifacts in the Jin vault were giving us the connection to blind girl+ice stone.
-How Bu-Yeon was saved by Jang Kang with the help of the ice stone also fits - ice stone is all about souls after all.
-Songrim founder's poem and Uk's words kinda alluded to this theory.
So in the end, this can go either way:
- After defeating the chaos Bu-Yeon's soul will return to the ice stone, ice stone will be destroyed and its power will return to the skies. Naksu stays in her body.
-Bu-Yeon will have enough of Naksu occupying her body and Naksu will be ejected into the ice stone. Then part 2 can focus on bringing her og body back somehow (this one is rather weak though).