Yes, it's another first-book-in-a-trilogy, sorry about that!
The Copper Promise is pretty standard sword and sorcery fare, done well and with a lot of work on the world-building, rather than just being pseudo-medieval Europe (though there are certainly significant elements of that still around in the order of knights to which one of our protagonists used to belong, for example).
We first meet one of the main characters while he's being tortured for information he doesn't have, namely the location of his family's secret vault; next time we meet Frith he is carrying the aftermath of that experience and hires our other two main characters (mercenaries called Sebastian and Wydrin) to help him break into the Citadel. This is a place that used to belong to the mages, who then sealed it with both that world's gods and their own magic inside, which they sacrificed to make that happen. Frith is, understandably, looking for revenge against the people who tortured him and killed his family and sees the acquisition of that magic as his way forward.
Of course, because this is a 500-page book and only the first of a trilogy, that doesn't work out quite as planned. Not only does Frith gain magic, which he doesn't know how to control, one of the gods is released in the form of a massive dragon. The dragon in question has also been busy during her captivity brooding a bunch of female warriors who are pretty much unstoppable and so the surrounding territory is doubly in trouble. The only possible chink of hope where the brood warriors are concerned is that Sebastian has also played a part in their coming to life and as the book goes on, we see some of them becoming individuals and starting to question why they are destroying everything.
Naturally, one of our mercenaries also has a troubled past (which I guessed the likely direction of once the words 'celibate all-male order of knights' got mentioned) and when our heroes are separated - Frith to try and gain mastery of his magic, Sebastian wanting to try and save his former colleagues and Wydrin to semi-successful thievery and a trip to sea with her brother) - the story does drag a little as they all cover a lot of ground, both physically and metaphorically. Their individual conflicts become much more interesting, in my opinion, when they are butting up against each other rather than bemoaning their circumstances alone.
If there is one minor quibble I have about the likely direction of a storyline, it's that The Copper Promise seems to be setting up Wydrin and Frith for a relationship, which I was really hoping wasn't going to be the case. As is mentioned more than once during the book, Frith is 'a bit of a shit' and while it's clear that Wydrin finds Frith physically attractive, which she comments on a couple of times, I just didn't see the groundwork as really being there. Still, I could still be pleasantly surprised about this (or not, we'll see!) in The Iron Ghost, which is the sequel, with book 3 still to be published.