Phoenix Blood by Jenny Schwartz

In a grim biker bar with wizard mercenaries on her tail and a “found” amulet around her neck, Sadie Howard needs a miracle. What she gets is the man who broke her heart nine years ago.

Marcus Aurelius is a changed man, in more ways than one. The preppy, confident medical school student is now a hard-bitten, magic-wielding assassin. He’s also a man on a mission. He has debts to pay and old wrongs to right before he dies—sometime this week.
As the secrets of Marcus’s heart are revealed, Sadie learns that nothing is as it seems and that the man who broke her heart also saved her life and paid in agony for her freedom.

With wizards trying to kill her, phoenix blood burning in Marcus’s veins, and a villain who’ll stop at nothing to acquire the amulet Sadie has promised to a friend, their roadtrip is a one-way ticket to extraordinary adventures. The question is, who will survive?