The Serpent's Throne: A spicy dark fantasy romance (Bloodlines) by Kathryn Ann Kingsley

I swore I’d kill Raziel Nostrom. Now he owns my heart… After a shattering betrayal, Nadi and Raziel flee into the Wild—the endless labyrinth of caves that is home to the fae. Nadi trained for years to infiltrate and kill a ruthless family of vampires. Now she faces an even deadlier challenge—returning to her own family with their most hated enemy by her side. In a realm of flickering lights and terrifying monsters, the desire that binds her to Raziel only grows stronger. Enemies and allies wish to tear them apart, but she still craves his razor-sharp smile, his touch, his bite… In the city above, Raziel’s siblings are building a new world, where vampires rule with no restraint and no mercy. Soon their power will be untouchable. Nadi and Raziel only have each other. Two shattered and broken things, in a world that wants them dead. Two perfect assassins, sharpened to a killing edge and hungry for revenge. Read more