Heart So Fierce and Broken (The Cursebreaker Series) by Brigid Kemmerer

In the sequel to New York Times bestselling A Curse So Dark and Lonely, Brigid Kemmerer returns to the world of Emberfall in a lush fantasy where friends become foes and love blooms in the darkest of places.

Find the heir, win the crown.
The curse is finally broken, but Prince Rhen of Emberfall faces darker troubles still. Rumors circulate that he is not the true heir and that forbidden magic has been unleashed in Emberfall. Although Rhen has Harper by his side, his guardsman Grey is missing, leaving more questions than answers.

Win the crown, save the kingdom.
Grey may be the heir, but he doesn't want anyone to know his secret. On the run since he destroyed Lilith, he has no desire to challenge Rhen--until Karis Luran once again threatens to take Emberfall by force. Her own daughter Lia Mara sees the flaws in her mother's violent plan, but can she convince Grey to stand against Rhen, even for the good of Emberfall?