Enchanted Queen by Tricia Wentworth

Prince Keiran is wallowing in Nerede when his brother arrives with news: after a decade of silence, Dra Skor has finally returned a letter. To avoid a war his healing country cannot endure, Keir knows he must travel to Dra Skor. Despite the danger, he is determined to do whatever it takes to fix the damage caused by his own tyrannical father.
But when he arrives in enemy territory, Keir’s carefully crafted plans are quickly turned upside down. Expecting another dictator, Queen Estalena is nothing like he imagined. And while Keir showed up to avoid a war, he seems to have walked right into one already simmering within Dra Skor.
Who is responsible for the quickly escalating civil war? And with a threat to the queen’s reign looming, can Keir figure out how to unlock their poisoned Enchantments before it’s too late?