"Until Death We Do Part" by Sherrilyn Kenyon
"Over five hundred years ago, Esperetta's soul was bound to her husband's by dark magic, and when Velkan became a Dark-Hunter, to her horror, she became immortal as well. Now, they must come together to fight an old enemy...and the passion that threatens to consume them once more."
Five hundred years ago, a young novice named Esperetta is saved from a vicious attack by the Romanian warrior prince, Velkan Danesti. He takes care of her afterwards, and they soon fall in love. Unfortunately, Esperetta is the daughter of Vlad Tepes (aka Dracula), and Velkan's father is Vlad's greatest enemy. Like in Romeo & Juliet, they are married in secret, but eventually Esperetta and Velkan must fake their deaths to escape their fathers' wrath and live happily ever after. Their plan goes awry when the two are actually killed by Vlad. Velkan is raised from the dead as a Dark-Hunter, and because Esperetta's soul was bound to Velkan's when they married by sorcery (unbeknownst to her), she too is raised from the dead and becomes immortal. Before Velkan can explain their fate to her, Esperetta witnesses Velkan killing her father. She runs away in terror, and so begins 500 years of misunderstanding. Fast forward to the present day: Esperetta (now called Retta) still hates Velkan because she believes he killed her father in cold blood. Velkan still hates Esperetta because she did not trust him and believed him a monster. As an old enemy surfaces that threatens them both, Velkan's servants lure Retta back to Romania in an effort to reunite them. Will they be able to forgive each other before it's too late?