Iron and Magic by Ilona Andrews

No day is ordinary in a world where Technology and Magic compete for supremacy…But no matter which force is winning, in the apocalypse, a sword will always work.
Hugh d’Ambray, Preceptor of the Iron Dogs, Warlord of the Builder of Towers, served only one man. Now his immortal, nearly omnipotent master has cast him aside. Hugh is a shadow of the warrior he was, but when he learns that the Iron Dogs, soldiers who would follow him anywhere, are being hunted down and murdered, he must make a choice: to fade away or to be the leader he was born to be. Hugh knows he must carve a new place for himself and his people, but they have no money, no shelter, and no food, and the necromancers are coming. Fast.
Elara Harper is a creature who should not exist. Her enemies call her Abomination; her people call her White Lady. Tasked with their protection, she’s trapped between the magical heavyweights about to collide and plunge the state of Kentucky into a war that humans have no power to stop. Desperate to shield her people and their simple way of life, she would accept help from the devil himself—and Hugh d’Ambray might qualify.
Hugh needs a base, Elara needs soldiers. Both are infamous for betraying their allies, so how can they create a believable alliance to meet the challenge of their enemies?
As the prophet says: “It is better to marry than to burn.”
Hugh and Elara may do both.