The Traitor Son Cycle #4: A Plague of Swords book by Miles Cameron

One enemy has fallen ... a greater one remains ... now it's war The Red Knight withstood the full might of his enemy and won the day. In a victory which will be remembered through the ages, he brought disparate factions together and turned them into allies against a more powerful foe than they had ever seen. Now, he will need his allies more than ever. Because behind one adversary hid another--one with allies of their own--whose goal was never to destroy Alba, but to distract the Kingdom while achieving his true aim. And whatever it is, it's probably not in the Red Knight's interest. With one army defeated, now the Red Knight must fight again--and for every one of his allies there is a corresponding enemy. Spread out in different lands, and on sea, it will all come down to one last gamble. And to whether or not the Red Knight has guessed their foe's true intentions. With each throw of the dice, everything could be lost.