Seize the Night (Dark-Hunter #6) by Sherrilyn Kenyon

Dark-Hunter: an immortal warrior who has traded his soul to Artemis for one moment of vengeance on his enemies. In return, they swear to spend eternity protecting mankind from the daimons and vampires that prey on them.

Valerius isn't a popular Dark-Hunter - he's a Roman, which means that the largely Greek Hunters have a major grudge against him and his civilization for superceding them.

To make things worse, he's very conscious of his aristocratic background and breeding. So it serves him right when he runs into Tabitha Devereaux. She's sassy, sexy, and completely unwilling to take him seriously. (Not to mention Tabitha is also the sister-in-law of Kyrian, a former Dark-Hunter and Val's mortal enemy.)