I started watching with no expectations other than "I'm probably going to hate this one", but after watching 20 episodes I can say that I actually enjoyed myself. There are also some things that I found really annoying about it as well, but I wouldn't say they ruined my experience entirely. I'll list what I didn't like first.
- The worldbuilding isn't very solid and it shows on multiple occasions. The magic systems of Daeho are never properly explained, which allows a lot of freedom for the writers to get creative with the events, but also lots of plot holes. There were multiple moments when I thought "wait, they could do that all along?!", and also moments when I wondered why didn't certain characters do anything.
- For a show that is supposed to be based on magical swordmasters, there's not many fights overall, especially for the main characters. It's even worse if we consider that the FMC is supposed to be one of the most powerful mages in the country AND an infamous assassin.
- The introspection of the characters was pretty awful. I could have ignored this if it was just for the secondary roles, but Naksu/Mu Deok herself is never explored as a character all that much. Since she's a protagonist I was expecting better for her.
- Some of the interesting ideas that were introduced during the first episodes were never used later on. One of them is the idea of the Four Seasons mages: I was looking forward to see them shine each in their own way, or at least to see an interesting synergy between the four of them in some fight, but none of that happened. It made me wonder why they got that introduction in the first place.
- The female characters are, for the most part, all imprisoned in the same old stereotypical roles. I was looking forward to see some beautifully written female characters, but they all resulted pretty flat. Mu Deok herself wasn't properly explored even though she's the main character, but she's also the only female character who is an actual swordswoman. Jin Cho Yeon's sole personality trait is her wish to be a wife, Kim Do Ju is just a parental figure, and I'm still trying to understand what was Heo Yoon Ok's reason to be in the story at all. So Yi seemed kind of interesting, but she got so little time on the screen that even her story turned out unsatisfying.
- I'm SO TIRED of love triangles, seriously. They weren't necessary at all. I'm starting to think that the writers include them in the plot just to get on everyone's nerves.
- I'm not mentioning anything specific here so that I don't spoil anything, but the resolution of many sides of the story were very unsatisfying in the end. I felt like some characters were given greatly important roles for no real reason, and also that some other crucial situations of the story got resolved very quickly and quietly. The last two episodes in particular felt extremely rushed compared to the rest.

On the other hand, there were multiple things that I liked.
- First of all, the themes of the story were portrayed pretty well: the struggle for power and for survival, life-or-death situation, the will to sacrifice something important in exchange for something else that is just as important... all these themes keep coming back in multiple situations of different characters and they were pretty consistent.
- The idea of a dangerous mage being stuck in a weak body and the son of a powerful mage who can't use his powers needing to team up was VERY compelling. The dynamic between the two of them was also amazing, with Mu Deok being a very strict, even cruel master and Jang Uk being a talented but also terribly spoiled disciple. They made a great duo and also complemented each other very well. I would have loved them even without their feelings being remotely romantic - I believe they worked just fine together as master and disciple. The way Jang Uk managed to develop his skills over time was also very entertaining, making me root for him all the way.
- Mu Deok is a merciless assassin, but she's also smart and good at acting as a certain role. The way she switched between being a meek and pandering servant to a strict and firm master was fascinating, I always looked forward to see her on the screen.
- The dynamics between other characters were also very good and endearing. The friendship between Jang Uk, Seo Yul, and Park Dang Gu was lovely to see in their interactions, and the tenderness between Park Jin and Kim Do Ju was another favorite of mine (along with their bickering). Jang Uk and Go Won were also a duo I didn't expect to love as much as I do.
- I believe that every single actor was chosen perfectly, really. Everyone did amazing. Jo Jae Yoon was the perfect antagonist, charismatic and scary at the same time. Props to Jung So Min as well for being amazing in her role, despite how difficult it was.
- Last but not least, this drama is visually amazing. The sets, the props, the costumes, and even the styling of the characters is varied and always beautiful. I really appreciated how characters had both long and short haircuts, even with different colors and accessories, instead of the usual period drama styles that can be seen everywhere else. I want to shake hands with whoever decided to give Yoo Joon Sang a beard: I kept thinking how handsome he looked every time he was on the screen.

My conclusion is that, while it's not a perfect drama nor a masterpiece in any way, it was a fun watch. If sword magic is your thing, you will have fun - if not, you might not find anything worth your time here. I'm looking forward to see if the second part of the story makes me change my opinion.