What about this Theory:

Im on board with this theory and how they could bring Cho Yeong=Naksu back , anyways this is Daeho anything is possible in a fantasy world

Introduction of characters,Uk trying to burn book Words of the Heart but this returning back 
https://www.youtube.com/watch?v=Q_UCdSCLqXg&ab_channel=tvNdrama

From the book of Words of the heart

https://twitter.com/ramblingsofafan/status/1554081954945007616

The passage "You cannot grasp water, but it is inside the soil. You cannot grasp fire, but it is inside the tree... If you let them be they will exist everywhere." It's almost telling of how Nasku's body can be revive, because the ice stone has that ability to gather what's left of Nasku's body (ashes that has steep in the soil?) and turns it back to it's original form (since it has the ability to revive human, the ability to turn water to fire, stone, than ashes ect..)

 imagine:

I have a small theory, but it isn't much, also not sure if this was mentioned before. 

I think that the reason that Jin Cho-yeon's father took out the bit of DNA snake from Cho-yeon (not sure what else to call it) is to give to the fake Bu-yeon (don't remember the name) so that she can open the door to the secret room in Jinyowon. 

I was thinking this because the mother would almost be certain to check if the blind girls Jin-Mu brought are Bu-yeon by getting them to try open the door. So they would have to figure a way for fake Bu-yeon to open the door. 

Yes, this theory is around here since the episode.  Its a great  possibility, i think you could be right.  

 mellody:
 As a person who has mastered Hwansu, Jang Gang returned to his own body without problem, but Cho Chung ran wild.

As far as we know, Jang Gang had NOT mastered Hwansu 20 years ago.  He used Soul Ejector in order to make the swap with the King.  But without a doubt, he was somehow involved in Cho Choong's death (whether intentional or accidental)  and when Naksu learned that, it will be another [CONTRIVED] obstacle between her and Jang Uk.

 wanderee:
Master Lee saying that Cho Chung’s daughter (the show implied it‘s Naksu through “obvious” clues) is dead could be a hidden fact. Besides, Master Lee knows it all. That can be an evidence that there are no two souls in one body. And he checked Mudeok’s body on their first encounter. Does that mean he didn’t see Cho Yeong’s soul in there? 

Master Lee's comments are the one reason why I think the One Soul/Split Personality theory is very plausible.  

W/r/t to the stone basin and constellation ceiling,  Bu-Yeon  definitely was a Chisu level mage.  Maybe it's also connected to  whatever ice stone is w/in her, but the Jin Mu's first assumption was that a Chisu level mage had done responsible for opening the constellation ceiling.

When CP asked Yul to fight Uk, he said Yul is his cousin. Since CP and Uk are cousins, Uk and Yul are cousins too? They just have not known it yet. CP's family name is Go. Yul's family name is Seo. May be Yul's mother's family name is Go. Just a thought. ?

 wanderee:
It can’t be because he mastered Hwansu. Naksu, who mastered Hwansu (she soul shifted without the soul ejector in ep1), is still afraid of becoming petrified.

I don't remember her saying that she mastered Hwansu. She said she mastered all three levels, which means she's still at Chisu level, but at the highest state.

 imagine:
I was thinking this because the mother would almost be certain to check if the blind girls Jin-Mu brought are Bu-yeon by getting them to try open the door. So they would have to figure a way for fake Bu-yeon to open the door. 

Yes, it seems to go down this route. There was a photo posted online, and I saw So-Yi aka Fake JBY in new clothes with Madam and JCY, so yeah.....

 mellody:

When CP asked Yul to fight Uk, he said Yul is his cousin. Since CP and Uk are cousins, Uk and Yul are cousins too? They just have not known it yet. CP's family name is Go. Yul's family name is Seo. May be Yul's mother's family name is Go. Just a thought. ?

No. 

Seo Yul's father, the General Seo who lives away, is the brother of the Queen (who is now soul shifted). The real Queen is Seo by birth. 

The Royal family is Go. So the Kings are Go. 

Uk is a Jang. His biological father is a Jang. But if you consider that he is also son of the deceased King he could be a Go. Then he could be CP cousin. 

I think you were talking about Uk's Mother? As far as we know she is not a Seo.  Do Hwa is from other fortress, but of course she could be relative to Master Heo Yeon who bring her and he is relative of Seos. But if  Uk was Yul's cousin I think they would have mentioned it already. 

 

 Kathryn_51:
 Bu-Yeon  definitely was a Chisu level mage.

I feel like Jang Uk and Bu-Yeon are born with the Hwansu-level energy already. This can explain why Bu-Yeon was so powerful even as a child. Since Jang Uk's energy gate was blocked, he couldn't bring out his full potential and would take some time to reawaken his power like Master Lee said. They are both born from Ice Stone anyway, so it's like their birthright already.

 mellody:

When CP asked Yul to fight Uk, he said Yul is his cousin. Since CP and Uk are cousins, Uk and Yul are cousins too? They just have not known it yet. CP's family name is Go. Yul's family name is Seo. May be Yul's mother's family name is Go. Just a thought. ?

Yes, the Queen is from the Seo family, Seo Yul's father's sister, which makes them cousins. Jang Uk is the former King's son, who was the older brother of the current king. So yeah, that makes them cousins too.

 Cau:
Seo Yul's father, the General Seo who lives away, is the brother of the Queen (who is now soul shifted). The real Queen is Seo by birth. 


 Fujihara Tomomi:
Yes, the Queen is from the Seo family, Seo Yul's father's sister, which makes them cousins. Jang Uk is the former King's son, who was the older brother of the current king. So yeah, that makes them cousins too.

Thank you very much to both of you for the clarification. Now I know the cousin relationship between CP and Yul is from the Queen (whose body is currently inhabited by the fake queen/Choi). I incorrectly thought the cousin relationship is from Yul's mother.

 Fujihara Tomomi:

I feel like Jang Uk and Bu-Yeon are born with the Hwansu-level energy already. This can explain why Bu-Yeon was so powerful even as a child. Since Jang Uk's energy gate was blocked, he couldn't bring out his full potential and would take some time to reawaken his power like Master Lee said. They are both born from Ice Stone anyway, so it's like their birthright already.

Yes, the Queen is from the Seo family, Seo Yul's father's sister, which makes them cousins. Jang Uk is the former King's son, who was the older brother of the current king. So yeah, that makes them cousins too.

I dont rember who said  it, but the person said that is only 2 person that have reach Hwansu. One it ofc master Seo Gyeon and they didn't mention the other person. The next person that's are gonna reach it is Uk(ofc he is the hero). base on ep 9 Jang Gang haven't reach Hwansu yet, maybe I have reach it and decided to return. 

My wild theory is that Jang gang are gonna teach Uk before he die! Why I think Jang Gang have to die is bc you can not have a line up with Jang Gang, Master Lee, Park Jin, Yul and Uk vs Shaman Choi/the Queen and Jin Mu. Its too easy for the "good side".

 RickRoss:
My wild theory is that Jang gang are gonna teach Uk before he die! Why I think Jang Gang have to die is bc you can not have a line up with Jang Gang, Master Lee, Park Jin, Yul and Uk vs Shaman Choi/the Queen and Jin Mu. Its too easy for the "good side".

Jin Mu is creating his own army of mages by soul shifting powerful (and older) mages into younger bodies.  

 mellody:


Thank you very much to both of you for the clarification. Now I know the cousin relationship between CP and Yul is from the Queen (whose body is currently inhabited by the fake queen/Choi). I incorrectly thought the cousin relationship is from Yul's mother.

There is a discussion topic that have descriptions of the characters and translations. its a very useful guide. :)

https://mydramalist.com/discussions/can-this-person-be-translated/73883-aos-character-descriptions-and-relationships-venue-descriptions-spoilers

 Cau:

There is a discussion topic that have descriptions of the characters and translations. its a very useful guide. :)

https://mydramalist.com/discussions/can-this-person-be-translated/73883-aos-character-descriptions-and-relationships-venue-descriptions-spoilers

Thank you so much 

 Kathryn_51:

As far as we know, Jang Gang had NOT mastered Hwansu 20 years ago.  He used Soul Ejector in order to make the swap with the King.  But without a doubt, he was somehow involved in Cho Choong's death (whether intentional or accidental)  and when Naksu learned that, it will be another [CONTRIVED] obstacle between her and Jang Uk.

Maybe Jang Gang helped Cho Choong to souls shift for what ever reason (may be he died and Jang Kang couldn't help but revive him using ice stone or something) and as a result he ran wild and killed his family. And Jang Kang couldn't even find his daughter. May be that's why he's apologetic to Cho family.

 Kathryn_51:

Master Lee's comments are the one reason why I think the One Soul/Split Personality theory is very plausible.  

W/r/t to the stone basin and constellation ceiling,  Bu-Yeon  definitely was a Chisu level mage.  Maybe it's also connected to  whatever ice stone is w/in her, but the Jin Mu's first assumption was that a Chisu level mage had done responsible for opening the constellation ceiling.

I think Bu Yeon lives only because there's an ice stone / half of ice stone inside her. After all she was dead inside the womb and Jang Kang somehow revived her. In a recent episode the queen revives the dead Eunuch Kim by placing  ice stone inside him. Maybe Jang Kang did the same with Bu Yeon.  

But  I still don't understand why Bu Yeon's and Naksu's souls can co-exist within Mu Deok.