In an attempt to piece together the story, I rewatched some prior episodes and compiled below some key info that might have been overlooked with each new episode and revelation:
1. Episode 1. Right at the start of the series, ML's dad had been supervising the construction of Fengshan Altar at the borders of Dayong and Dongxia for 3 years, and at the same time supervising the underground digging, searching for Gui Seal, on the instruction of the Dayong Emperor.
- He asked his assistant on the underground situation and when updated that the soldiers had dug more than a thousand foot deep, he said that should do and told the assistant to tell the soldiers to stop digging. Meanwhile underground, the soldiers had uncovered a deep pit, and something must have been released from the pit, making the soldiers unconscious. ML's dad went into the pit with his assistant and saw a maze-like chamber with an upside down pagoda, and a shining green object inside, and sinister troops seemingly guarding something precious.
- Marquis Pingjin was stationed at the borders for 10 years, and had lost to undead troops before he finally defeated Dongxia, and the Emperor must have known about the presence of the undead troops and the Gui Seal. Also see point 4 - The Emperor's father was killed by the Gui Seal.
- The Gui Seal belongs to the Dongxia Kingdom, and the Dongxia imperial folks should know how to use the Gui Seal.
2. Episode 1 - Opening credits. Did you notice the sequence of the cast list and find any peculiarity? Dongxia Queen was listed 5th, right after Marquis Pingjin (4th), and ML's mom was listed 7th, right after the Dayong Emperor. Dongxia Queen has not appeared and ML's mom's appearance was brief in the 1st episodes and in flashbacks only. A picture of the Masked Man was also shown in the cast listing, following by the ML's dad. Dongxia Queen, ML's parents (especially his mom) and Masked Man should be crucial for the remaining plot.
3. Episode 3 - Puppet show. Current Dayong Emperor has no son. His father, the late Dayong Emperor was buried with the current Dayong Emperor's mother, Lady Li. The recently deceased Empress Dowager has a son Linzi Duke who is still alive, had been stationed at the borders, and would like to return to the capital for his mother's wake. I dont recall seeing his appearance. Could he be the Big Boss behind the Masked Man?
4. Episode 20 - Yongrong Duke. He is the brother of Dayong Emperor, and knows the existence of the Gui Seal, and he said that his father (late Dayong Emperor) was killed by the Gui Seal.
5. Episode 21 - The female storyteller, FL's best friend, is the daughter of Zhao Bingwen. She has helped ML when Zhao Bingwen suspected ML's motive when he found him in his study room alone. Zhao Bingwen is unlikely the Masked Man. But it is not clear why his daughter helped ML.
With the above points, it should be safe to conclude the following are after the Gui Seal:
1. Marquis Pingjin
2. Eunuch Cao, in collaboration with Dongxia Queen, only because he need a powerful backer, to remain powerful. His current power comes from the current Dayong Emperor, and this Emperor does not have kids.
3. Dongxia Queen - The Gui Seal belongs to her, and she knows the secret of the Gui Seal and how to use the Gui Seal. As she has not appeared, it is unclear if she harbours thoughts to revive the Dongxia Kingdom with the Gui Seal.
4. Dayong Emperor - Could he be searching for immortality from the Gui Seal, since he does not have kids?
5. Linzi Duke - He is the son of the Empress Dowager, but he is not the Emperor and only the Duke and had been stationed at the borders. He could be harbouring thoughts to use the Gui Seal to command the undead troops to overthrow the current Dayong Emperor.
6. Masked Man - The fact that he masked himself could be he is someone well known or of certain official status and does not want to be recognised. He could be using the ML to help him get rid of Marquis Pingjin and Eunuch Cao, It remains unclear his motive for finding the Gui Seal, but could be someone working for Linzi Duke?