The Dragon Queen by Penelope Barsetti

Talon says the most horrible thing—that he doesn’t expect to survive the war.

It’s an outcome I don’t want to consider. My whole life has been an endless reign of suffering until I found him. He was the one who caused this misery—but he’s also the one that healed it. Happiness is a terrifying joy, because once you have it, all you can think about is the moment you lose it.
It’s been decades since Talon has been to the Southern Isles, the place that had been his home for most of his life. Ethan says the place has changed, that the citizens are locked inside the kingdoms, that sailors can’t get into port without an inspection, that the people are starving from a mysterious plague.
Before the battle can commence, we need to know what we’re up against, so I volunteer to enter the castle as a maid and report everything I know. I’m not there for a week when I see that Barron is exactly the swine that Talon says he is. It’s there that I discover a horrid list of truths.
It’s there that I discover the Southern Isles has been destroyed by a dictator—and Barron is already prepared for war—because he knows exactly who the Death King is.