Born of Darkness by Lara Adrian

As a former assassin in the Hunter program, Asher is one of a small group of Gen One Breed vampires who survived the horrors of a madman’s laboratory and the cruelty of the training that made him one of the most lethal beings in existence. Now, twenty years after his escape from those hellish origins, Ash is a loner whose heart is as cold as his skills are lethal.

But when a beautiful, secretive young woman on the run from dangerous enemies crosses his path in the middle of a dark night in the desert outside Las Vegas, Ash is drawn into a deadly game where trust can be an illusion and love may be the sharpest weapon of all.