Midnight Unbound by Lara Adrian

As a former Hunter bred to be a killing machine in the hell of Dragos’s lab, Scythe is a dangerous loner whose heart has been steeled by decades of torment and violence. He has no room in his world for love or desire—especially when it comes in the form of a vulnerable, yet courageous, Breedmate in need of protection. Scythe has loved—and lost—once before, and paid a hefty price for the weakness of his emotions. He’s not about to put himself in those chains again, no matter how deeply he hungers for lovely Chiara.

For Chiara Genova, a widow and mother with a young Breed son, the last thing she needs is to put her fate and that of her child in the hands of a lethal male like Scythe. But when she’s targeted by a hidden enemy, the obsidian-eyed assassin is her best hope for survival… even at the risk of her heart.