Defy the Night by Brigid Kemmerer

The kingdom of Kandala is on the brink of disaster. Rifts between sectors have only worsened since a sickness began ravaging the land, and within the Royal Palace, the king holds a tenuous peace with a ruthless hand. King Harristan was thrust into power after his parents' shocking assassination, leaving the younger Prince Corrick to take on the brutal role of the King's Justice. The brothers have learned to react mercilessly to any sign of rebellion--it's the only way to maintain order when the sickness can strike anywhere, and the only known cure, an elixir made from delicate Moonflower petals, is severely limited. Out in the Wilds, apothecary apprentice Tessa Cade is tired of seeing her neighbors die, their suffering ignored by the unyielding royals. Every night, she and her best friend Wes risk their lives to steal Moonflower petals and distribute the elixir to those who need it most--but it's still not enough. As rumors spread that the cure no longer works and sparks of rebellion begin to flare, a particularly cruel act from the King's Justice makes Tessa desperate enough to try the impossible: sneaking into the palace. But what she finds upon her arrival makes her wonder if it's even possible to fix Kandala without destroying it first.