The Hemlock Queen by Hannah Whitten

In the second installment of New York Times bestselling author Hannah Whitten's lush romantic epic fantasy series, the glittering and dangerous world of the Sainted King's royal court is upended when Prince Bastian seizes control and a mysterious dark force begins to take over.
The corrupt king August is dead. Prince Bastian has seized the throne and raised Lore-a necromancer and former smuggler-to his right-hand side. Together they plan to cut out the rot from the heart of the royal court and help the people of Dellaire. But not everyone is happy with the changes. The nobles are sowing dissent, the Kirythean Empire is beating down their door, and Lore's old allies are pulling away. Even Prince Bastian has changed. No longer the hopeful, rakish, charismatic man Lore knows and loves, instead he's become reckless, domineering, and cold.
And something has been whispering in her ear. A voice, dark and haunting, that's telling her there's more to the story than she knows and more to her power than she can even imagine. A truth buried deep that could change everything.
With Bastian's coronation fast approaching and enemies whispering on all sides, Lore must figure out how to protect herself, her prince, and her country before they all come crumbling down and whatever dark power has been creeping through the catacombs is unleashed.