In this third volume of the loosely coupled trilogy - as the books can be read standalone - there are two protagonists: a con-man and a demon. Quite a lot of backstory about the demon is established first and he is probably the best realised character in the series as a whole, as a weakness of these novels is the characterisation. Astron has a genuine arc in the growth of his character. I enjoyed the parts where he appeared especially in his own domain. I also enjoyed the initial misadventures of him, Kestrel the con-man and Phoebe the female wizard when they go on the run and try to reach the archimage, in the first part of the book.
The story hinges around Astron's quest to resolve a conundrum which has been posed his master, the Prince who, in book 1, was outmatched by Alodar who subsequently became the archimage. (This third volume is set about 30 years after book 1.) A rival prince has set the Prince this puzzle and if he cannot resolve it, will destroy him. The Prince's situation becomes steadily grimmer throughout the story as Astron journeys from one realm to another, beginning with the realm where the series is usually set, to try to find the answer and obtain harebell pollen which another demon prince has demanded before giving them help. On the way he finds out more about humanity, experiences what it is to be human at one point, and gradually develops feelings for a fay ruler.
As with other books in the series, there are inventive contraptions, contrivances and in this case whole worlds, including one which has arcane board-game type rules. The biggest weakness in the story is the characterisation of the other characters, especially the women. In previous books the women have been almost cardboard cutout and the tendency continues here: there is an attempt to make Phoebe, whose role is love interest for Kestrel the con-man, more realistic by having her doubt her abilities - there is a lot of sexism in the wizards guild - but she does end up acting like a wimp most of the time, as does the female fay ruler who is the love interest for Astron.
Another issue is that I worked out pretty early on who was the real villain manipulating things behind the scenes. And some parts dragged rather, especially when they are almost literally sitting around for 10 or 15 days in the fay realm for no better reason, it seems, than to give Kestrel and Phoebe the chance to learn the fay language instead of Astron having to interpret for them. I would have enjoyed the book more if there had been more of the con-man tactics employed in reaching the archimage which happens too quickly after their initial misadventures: those were quite enjoyable. So on balance I would rate this 3 stars.