Unforeseen Mate by Delta James

In the hidden world of shifters, a hellhound rescues a forbidden human woman. Instead of returning her to those who sent him, he keeps her.

Hayden is the alpha hellhound of his pack. He never imagined that his life would intertwine with fate in such a profound way. Everything changed the day he was asked by someone to whom he owed a blood debt to rescue her. Her name was Fallon, and she had been kidnapped by an ancient dragon – a shifter who had taken her to try and force the compliance of one of his assassins.
Despite the task he had been given and the laws that forbid him from claiming her, Hayden claims her as his mate. There are consequences to such a union some of which could destroy the world as we know it.
As the evil that is the Shadow League threatens our world, our love grows. A battle awaits, and a powerful bond emerges. Will their love be the key to victory or the instrument used to destroy both of their worlds?