Wicked king by Holly Black

The Setup: You must be strong enough to strike and strike and strike again without tiring. The first lesson is to make yourself strong. 

After the shocking revelation that her younger brother Oak is the true heir to Faerie, Jude has done the unthinkable: she has bound the wicked King Cardan to her will, forcing him to serve as king while she rules from the shadows as his seneschal. For five months, she has secretly controlled the throne, keeping Oak safe and maneuvering through the treacherous politics of the faerie court .

The Conflict: But controlling a king who despises you is a precarious game. Cardan does everything in his power to humiliate and undermine her, even as his fascination with her remains undiminished . Meanwhile, threats close in from all sides: the Queen of the Undersea demands an alliance through marriage, Jude's bloodthirsty foster father Madoc schemes to reclaim power, and a traitor lurks among those she trusts most .

The Stakes: When it becomes clear that someone close to Jude means to betray her, threatening her life and everyone she loves, she must uncover the traitor before it's too late. Forced to navigate shifting alliances, assassination attempts, and her own dangerously complicated feelings for the king she controls, Jude fights to maintain her grip on power as a mortal in a world that wants her dead .

Key Themes: Political intrigue, betrayal, the weight of power, and a slow-burn enemies-to-lovers romance that builds toward one of the most devastating cliffhangers in young adult fantasy