Land of Shadow by Celia Aaron

Dr. Georgia Clark works to find a cure for the plague that has ravaged humanity and ended civilizations. Georgia’s sister matches her efforts, but instead of seeking a cure through science, Juno seeks salvation through power. When superhumans emerge from the shadows, their emissary Valen Dragonis offers what humanity has failed to discover on its own: an opportunity to survive.
Valen becomes Georgia’s only source for the lifesaving blood dubbed “Juno’s Miracle.” Focused on her work, she doesn’t realize his claws are sinking deeper and deeper as he traps her in his twisted grasp.

The cure and power. Pleasure and pain. Valen offers all this and more. But the vampires never intended to give their help for free…