A high fantasy that involves gods and an empire divided, with magical warriors fighting to serve the one they are sworn to, Circle of Shadows has an interesting magical system for its world-building, but the plot is quite predictable. This book had caught my eye because, well, magical ninjas. However, it does not really have much of Japanese culture in it beyond the warriors, some names, and the clothing is ambiguously Asian? The main characters, Sora and Daemon, are the top warriors in their graduating class, in an Academy where kids are trained to be the Empress' sworn warriors as well as the central law enforcement. They are young, and thus not entirely believed when they come back with a report (from their first mission) of the rise of a supposedly dead prince, who had caused a massacre a decade ago, and may be rising to challenge his sister, the Empress again.
Armed with new magic, the Dragon Prince Gin is recruiting warriors to his cause via mind control. The native magic of the warriors is to use mudras to get animalistic abilites like enhanced sight, speed and strength, but the new ryuu magic is much more versatile and expansive. Sora and Daemon first have to spy, then fight him with the help of whatever warriors are left untouched by the mind control, which has them at a disadvantage as ryuu magic is more powerful. Coupled with that, Sora has a personal stake in the matter as Gin was the cause of the nursery fire in which she lost her sister. Daemon, meanwhile has a mysterious past that gets hinted at the end of the book.
The magic of the book is its strong point, as it is explored in different aspects. The plotline, however, is predictable, with most twists clued in quite a while ago. Still, it benefits from engaging writing and a good pace, so you are enjoying the story nevertheless. The antagonist could be developed better, in my opinion, because right now he is just Generic Bad Royal #23.