Nexus by Madilyn DeRose

Three months ago, Lock cared about two things. Blood & power. While those remained constant, the current state of affairs was far from it. What should have been an exciting transition for his mate, Sam, has proven to be more difficult than he could have imagined. Every day she withdraws further into herself, hiding away from the world and from him. With her newfound magic becoming more volatile with every passing emotion, and the shadow drawing ever closer, longing to tear them apart, he must find a way to make her face the darkness inside of her, even if he has to force her hand.
There was one word to describe exactly how waking up as a vampire felt. Wrong. Lock told Sam she’d feel powerful and indestructible. Whole. All she felt was broken. She was a constant danger to those around her, a monster of proportions her mate couldn’t understand, even with the depraved depths of his mind. When trying to figure it out on her own continues to fail, there’s only one person she can turn to. He could help her or kill her; either way, she’d accept her fate, if only so that she couldn’t hurt anyone else.
Failure was a word Seraphim had long left in his past, but the last four years had included a long list of them. Nearly all of them were centered around one thing: a bad-tempered redhead with some supernatural claim on his blood. The only problem was that he wasn’t hers to claim. Regardless of what Sam might want from him, he’d all but sold his soul to something far more menacing than her. There’s only thing for him to do and it appears he’s found the perfect opportunity because she’s weak. Damaged. Lost. As he’s pulled in two opposing directions, he must consider the repercussions that come with each of his decisions. If he doesn’t take care of this problem, there are other, worse beings that could extinguish them all without hesitation or remorse.