Ah that's what it was! When I saw today's TWSSG Episode 24 10-minute Preview, I finally got it. OMG, the scriptwriter amazes me on this lead. Remember in Episode 16, when this member of the Gomul Village, who was also a loyal servant of the late King and also Damdeok, don't know his name, but here is his picture
So in episode 16, when he overheard Kiha and Hwuchwan elder's conspiracy about blocking Damdeok's army outside the royal city, he ran to send the message to Damdeok. But he did not escape Saryang's watchful eyes.
While he was preparing the horse to hit the road, he was caught by the Hwachwan "ninja".
But the story stopped right here, I knew he was in trouble with the Hwachwan people, but surprised to see he was all safe and sound when Damdeok came back.
He was with the king even till episode 23, appearing as a loyal servant. I have been wondering what's up with this small section of him being captured by Hwachwan ninja, either the scriptwriter's mind was going wild, or there WAS something she was preparing, but very very cunningly ...
So the episode 24 preview gave me an answer - Aha! he was one of those "marked" spy of Hwachwan since episode 16, good thing he did not do much damage untill now.
But Hwachwan elder knows the best time to use his deepest spy. He made him stole Damdeok's son!
Oh boy, oh boy, scriptwriter Song Jina, you are very creative. I admire.
You can take my comment either way, serious or sarcasm, but I am actually pretty serious, this "spy" was indeed hiding too early and deeply. I've read some viewers had negative comments about the scriptwriter, in my opinion, it is fair to say she is darn good.