V.E Schwab 
Shades of Magic [PDF ebook] 
The Steel Prince #3.4

The fleet of the Red Army, led by Anatari magician Rowan, has swept across the coastal towns of the Blood Coast like a tsunami, putting to the sword all who refuse to swear allegiance and join his army. Now the only town that stands in his way is the garrison town of Verose, led by the Prince Regent, Maxim Maresh.Together with its people, Maxim and his men unite to defend the honor of the Maxim Empire against the threat of Rowan and his Red Army. Valiantly, they fight in bloody hand-to-hand combat as the streets burn and Prince Maxim is wounded in battle.Maxim discovers that three ships remain at the rear of the fleet conducting the attack and deduces that he will find Rowan on-board his flagship. Despite protestations from Isra, Maxim sets off alone to confront his nemesis there…