Seduced in Darkness (The Eventide #1). The Eventide

Rosalynn Monroe has until her twenty-fifth birthday to decide whether she lives or dies. Rosalynn may be one of the best slayers New York City has to offer, but she can't escape Michalis, the city's master vampire. Obsessed with Rosalynn, Michalis is determined to make her his queen on her twenty-fifth birthday unless she can thwart him. She's suffered his sexual games for years, and she's finally reached her breaking point.
Though her body delights in his carnal expertise, he challenges everything she holds dear. Her only hope is another slayer, Marcus Dale, a man who lives on the edges of society and seems to hold Rosalynn in low regard. Marcus makes her an offer that's nearly impossible to refuse, a chance to escape from Michalis for good. Torn between a fatal love and a seductive enemy, she must try to save her soul and defeat this vampire who won't take the word no for an answer.