There were 2 mischievous things in that room! The mirror and that upturned boots, the one that tripped youngest Jin daughter aka Barbie mage and end up breaking the jade. They don't seem to want Barbie to be with Jang Uk. I'm still with my theory that Mu Deok would marry Jang Uk by virtue that both are eldest in their fam and possibly the strongest mage families (well that's the reason Uk was to marry Barbie mage - sorry I forgot her name but she looks like Barbie doll in all her get up, cute and pretty).

What if Jang Uk is not the son of the previous ruler? Or maybe the letter that Jang Kang received was fake and the king never spent the night with Jang Uk's mother? Crazy theory: How about the theory that the deceased king had a daughter and not a son? And it is Naksu. Mu Deok was kidnapped and blinded in her childhood because she witnessed some event - perhaps the king's secret meeting with Naksu's mother? As a child, Naksu was sent to an astrological center and raised by a magician there who protected her and knew who she was, so he was able to read her horoscope, which included the king's star.

Or is Mu Deok the daughter of the king? Maybe the king's first wife was Jin Cho Yeon's mother's sister? Unfortunately, she died when her child was born. A blind girl was born, whom the king refused to accept because of her disability? Her sister and her husband decided to raise her as their own daughter? She was kidnapped and missing.

Or maybe Mu Deok was fatally wounded when she was kidnapped. She was found by Jang Kang and taken to the astrology center. There her soul was placed in Naksu's body. And now the soul has returned to its body? There is no blue mark on Mu Deok's body but her eyes are blue? Do you know the saying that the eyes are a reflection of the soul, in the eyes of a person you can see its soul? Therefore, Naksu's soul was placed in them. Either it is her own body or Mu Deok let her enter her body to regain her lost sight?

As for the previous deceased ruler, if he didn't die, I don't think he is in the body of an eunuch. The person in it is only a spy that Jin Mu uses. It is probably the king's eunuch who informs him of what the king is doing and what concubine he is meeting with. Jin Mu and the current queen want her son Go Won to be the next ruler. The current king plans to have another son, so that this son in his chart will have the king's star. As for the previous ruler, it could be Jin Mu's right hand man. I think I just came up with a brilliant theory. What if the previous ruler had planned everything together with Jin Mu. They decided to use the naivety of Jang Kang, who was the only one who mastered the alchemy of souls. The king did not want to leave a descendant behind, but to receive another life and another chance to be the ruler again. He is to be his star. Everything was planned in advance. As a last resort, the king is to move into a different body. It will probably be Go Won's body. Jang Uk is not the king's son because he would never try to kill him. What if the king's horoscope was faked and never contained the king's star?

I have another brilliant theory. Jang Uk was not born with the star of the king because he is his son, but because he will be elected in the future by the assembly of mages, after the body of Go Won will be taken over by the previous ruler. As you know, the Jin family has a pledge that gives them the right not to obey the king's order and even overthrow him if all the mages in Daeho anonymously vote for it. It is likely that Jang Uk will save Daeho, defeat his enemies and he will be chosen the new ruler.

Don't know what it means, but when the younger Jin sister goes into the vault to get the Jade stone, the image she sees in the mirror (after getting tripped by the jar) looks exactly like Naksu

https://imgur.com/a/NAtMKpG

Then when Mu Deok goes into the vault (however she got there, tba I'm sure) she sees herself in the mirror.

If the elder Jin sister, Naksu, and Mu Deok are really the same people (or even if they aren't), there better be a good explanation.  And I still have  hard time with soul-shifted parentage, if that's actually what's going on with Jang Uk.

 Kim Hee Soo:

There were 2 mischievous things in that room! The mirror and that upturned boots, the one that tripped youngest Jin daughter aka Barbie mage and end up breaking the jade. They don't seem to want Barbie to be with Jang Uk. I'm still with my theory that Mu Deok would marry Jang Uk by virtue that both are eldest in their fam and possibly the strongest mage families (well that's the reason Uk was to marry Barbie mage - sorry I forgot her name but she looks like Barbie doll in all her get up, cute and pretty).

What if Barbie and Mu Deok are siblings? It will complicated a lot of things 

 Conservative_Torch:

What if Barbie and Mu Deok are siblings? It will complicated a lot of things 

Yes, they may be sisters. At first, Mu Deok will probably be negative towards her, because she still thinks that her father was killed by the representatives of four families, but with time she will become attached to her. I think that even for this reason she will distance herself from Jang Uk.

 Thecla:

Don't know what it means, but when the younger Jin sister goes into the vault to get the Jade stone, the image she sees in the mirror (after getting tripped by the jar) looks exactly like Naksu

https://imgur.com/a/NAtMKpG

Then when Mu Deok goes into the vault (however she got there, tba I'm sure) she sees herself in the mirror.

If the elder Jin sister, Naksu, and Mu Deok are really the same people (or even if they aren't), there better be a good explanation.  And I still have  hard time with soul-shifted parentage, if that's actually what's going on with Jang Uk.

The image we see in the mirror is  Jin Cho Yeon.  The younger daughter of Jin´s family.  Did i miss something?  We can see its Arin. 

 Izyda:

What if Jang Uk is not the son of the previous ruler? Or maybe the letter that Jang Kang received was fake and the king never spent the night with Jang Uk's mother? Crazy theory: How about the theory that the deceased king had a daughter and not a son? And it is Naksu. Mu Deok was kidnapped and blinded in her childhood because she witnessed some event - perhaps the king's secret meeting with Naksu's mother? As a child, Naksu was sent to an astrological center and raised by a magician there who protected her and knew who she was, so he was able to read her horoscope, which included the king's star.

well, i think the real identity of Jang Uk together with the real identity of Mudeok and Naksu are the great mistery of the series.  But so far about Uk we know :

- The King spent the night with Uk´s Mother in the body of Jang Kang.  The series show us this. 

- There is the prophecy of the star in the moment of Uk´s born. His father know this.

- We dont know exactly what his father believe because he didnt tell all for Park Jin, but he didnt want to kill Uk,  but at sametime block his energy.

- Uk is very powerfull indeed. VERY powerfull. Can learn every magical stages very fast. He is a mage.

From this info, for me he is son of Jang Kang and the King at same time.  He is a mage but is destined to be the King. 

 Thecla:

Don't know what it means, but when the younger Jin sister goes into the vault to get the Jade stone, the image she sees in the mirror (after getting tripped by the jar) looks exactly like Naksu

https://imgur.com/a/NAtMKpG

Then when Mu Deok goes into the vault (however she got there, tba I'm sure) she sees herself in the mirror.

If the elder Jin sister, Naksu, and Mu Deok are really the same people (or even if they aren't), there better be a good explanation.  And I still have  hard time with soul-shifted parentage, if that's actually what's going on with Jang Uk.

That was Jin Cho Yeon dear

@Cau                                                                                                                                                                                                                                                      - The King spent the night with Uk´s Mother in the body of Jang Kang.  The series show us this.                                                 I'm not sure there was a love scene. I think Jang Kang somehow stopped the king, and he doesn't know it himself. Or it was just his imagination that made him believe it. Probably Do Hwa was already pregnant and the King found out about it through Jin Mu. All this was to serve only as a basis, because for me, the king did not want to have a descendant, but only he wanted to receive another life and reign again.                                                                  - There is the prophecy of the star in the moment of Uk´s born. His father know this.                                                                              Yes, the star of the king appeared in the sky when Jang Uk was born, but as for me it happened not because he is the son of the king, but because he will deserves it and the assembly of magicians will choose him as the new leader.

 Izyda:
  Yes, the star of the king appeared in the sky when Jang Uk was born, but as for me it happened not because he is the son of the king, but because he will deserves it and the assembly of magicians will choose him as the new leader.

That's the best idea I've heard from anyone.  Jang Uk earns it  - doesn't inherit it.  Very good!?

 Izyda:

Ok, we all know Jang Uk may be the son of the previous ruler. But in fact, it doesn't make any sense at all. Even if they exchanged souls, that body still belonged to Jang Kang. After all, the soul has no DNA ?! : D It's twisted.

He is genetically Jang Kang's son.  But I think the soul makes a person.  The king received the divination about his son being born under the king star and that led to Uk. The prophecy was right, the star was there when Uk was born.
Birth charts are all about timing. If not for the king, maybe Jang Kang wouldn't impregnate his wife at that moment.
I did post a theory concerning the alchemy of souls, but I don't think it's relevant in this case anymore. I don't think the writers will bother to explain the whole paternity thing, this is fantasy after all.