This is a very well done work! I admire the writing style, which is clear and engaging without any wasted exposition or unnecessary detail. The opening is somewhat cliche, but the action proceeds at a nice quick pace, so that really isn't a major problem. I do have some issues with the main character, Konstantin, however. He comes across as somewhat stilted and under-developed, the least interesting aspect of the story. There are two separate problems with this protagonist that I think you can correct to improve your story.
The first, less serious, problem is that we get almost no background on Konstantin at all. We have no idea who he is, why he is there, or what he is trying to accomplish within the larger plot. I've only read the first chapter so far, so I understand that you may be reserving most of these details for a reveal later on, but even in the very beginning the readers need something to frame this character. What makes this more frustrating is that you provide yourself with the perfect opportunity, while Konstantin and Scipio are sitting around their fire in the forest, to provide some very brief information regarding who Konstantin is. After all, this is when you reveal who Scipio is- which was very well done by the way. Having already evoked a "Roman Empire-esque" setting, informing us that he was a former legion commander wanted for treason is all we need to know about him at this point (and the hints you provide that there is something magical about this character are also well done). So you should take advantage of the moment and have Konstantin share some information about his background as well.
The other problem is more fundamental. Despite the fact that you are using the third person limited narrative style, and in spite of the fact that you largely succeed in "showing, not telling", I still feel that you arent sharing enough information about Konstantin's inner self. I get almost no feeling for what type of personality Konstantin has, whether he's an honest man or a dishonest one, is he brave or cowardly, and so on. Again, I've only read chapter one (which appears to be about 5-6 pages if it were a word doc), and you may be planning to reveal more about him later on, but I think even now the readers need some clue regarding what type of person the protagonist is (meaning what type of hero). Is he flawed but noble? Is he searching for something? Running from something? Is he keeping a secret?
This is going to be challenging because of a somewhat related issue (I guess this is problem number three) which is that your hero doesn't actually initiate anything. From the moment he opens his eyes in the prison, to the moment his companion is attacked in the forest, Konstantin only reacts to events. Narratively that makes sense given what is happening, but it does mean that you provide yourself precious few opportunities to show the readers who Konstantin is. Not only does he not seek to initiate anything, he doesn't even express any frustration at not being able to influence what is happening to him. The moment that this seemed least realistic was when Konstantin was being led to his own execution. He doesn't once complain at the unfair treatment he is receiving. Maybe he isn't a complainer, but still. Shouldn't he at least grit his teeth, or clench his fists, or something? I think you should show us what kind of man he is by giving him some feelings.
I hope this was helpful.
I really enjoyed reading your work and I strongly encourage everyone else to do so as well.
“There’s room for all of us here... But there’s no middle ground between ‘We belong here’ and ‘No you don’t.’