V.E Schwab 
Shades of Magic 
The Steel Prince #9

Exiled by his father to the coastal garrison town of Verose, hot-headed, arrogant, and untested Crown Prince Maxim Maresh went on to prove his worth by defeating the pirate queen Arisa, with the help of a new-found ally Isra, a member of the Arnesian garrison and a gifted solider and fighter in her own right.Following his victory, Maxim ignored his father’s subsequent invitation to return home and chose to remain in Verose, determined to prove to the soldiers in his charge that he was far more than just another spoilt prince and worthy of the honor of leading them. Learning of an annual magical tournament called the Night of Knives, that tested bravery, honor and courage, Maxim entered the contest and became the only person ever to complete the four deadly magical trials.Meanwhile in the waters off the Blood Coast, forces are gathering that could threaten not only Verose, but the whole of the Maresh empire…