Fairies of Death (Belfean Courts #1) by Victoria Liiv:
 
Is she the hunter or the prey?

Collecting life force is essential for death fairies' survival. To give it away is a death sentence.

Isay’s life drastically changed when she was forced into the Death Court of the Fae. Although one might expect to feel safe when being guarded like a Royal treasure, Isay is far from it. Her life force attracts the attention of every death fae. Her guardians might more successfully control their hunger, but no one is entirely immune to her appeal. In the midst of intrigue and fear, she is drawn to one of her protectors. Karmuth’s softer demeanor, and incredibly blue eyes, lull Isay to believe he poses less of a threat. She will learn in time that the opposite is true.

Karmuth has an arrangement with one of the courtesan’s in the court: sex for life force. He never considered why Sela offered the deal, collecting life force without killing his victims was the sole reason he kept going back to her. The arrangement never interfered with his personal affairs but an irresistible nature fairy may change that. Isay’s innocence and determination captivated him from the moment he first saw her. Her presence also awakened an insatiable hunger that Sela’s services failed to alleviate.

When Isay runs into danger head first, Karmuth must protect her from more than the Death Court and himself, but also a brewing conflict with an ancient enemy. Friends are not to be trusted, love is a distraction, and death is certain. But whose death?