Beneath the keep : a novel of the Tearling
(Book)

Series numeration from Fantastic Fiction, https://www.fantasticfiction.com/j/erika-johansen/.

The Tearling, founded as a utopia, has collapsed and reverted to feudalism. As the gap between rich and poor widens and famine threatens the land, rumors of a prophecy begin to spread: a great hope, a True Queen who will ascend and save the kingdom. Lazarus, a boy on the verge of manhood, is trapped in the underworld known as the Creche. Enlisted from his earliest days to kill without mercy, he has never seen sun or sky until a quest for vengeance propels him aboveground. Amid a royal court rife with intrigue and danger he meets Niya, Princess Elyssa's handmaid, who is not what she appears to be - and whose true identity will spell death if revealed. -- adapted from jacket