Trial of Hearts by Michelle Madow

She kept a deadly secret. He gave her his heart. Now the darkness between them might destroy everything.

Sapphire thought she could keep her vampire nature hidden from Riven—the ice-cold fae prince who’s slowly been melting for her. But when a winged creature attacks their camp and she loses control, drinking its blood right in front of him, the disgust in his silver eyes cuts deeper than any blade ever could.
Now, as they race to save Sapphire’s best friend Zoey from the sinister Night Court, every moment together is agony.
But there’s no time for heartbreak. The Winter King’s madness threatens to consume his court, and only an ancient potion can save him. To find it, Sapphire and Riven must venture into hostile territory, face deadly trials, and forge impossible alliances. All while Sapphire battles the bloodlust that nearly destroyed them once before.
As secrets unravel and war looms on the horizon, Sapphire finally discovers her true heritage and learns it may be the key to everything—or the catalyst that brings two courts crashing down.