Fate's Wrath. Altered Karma Series, #4

After a lifetime of saving others with science, a doctor must learn to save the man she loves the only way left available to her - by becoming the weapon Phoenix Corps never saw coming. She made a promise. Phoenix Corps just made sure keeping it would cost her everything. On the run with stolen evidence and a target on their backs, Fate and Cole fight their way across a broken landscape toward the one place they might finally be safe.
Every mile changes her. Every battle leaves a mark she can't wash away. The scientist who believed in the mission is gone - and what's rising in her place surprises even her. She came to study him. Now she's fighting beside him, for him, because of him - and the wrath driving her forward is the only thing keeping them both alive. She never imagined she'd become someone capable of this. But Phoenix Corps took everything from the people she loves, and Fate's wrath, once unleashed, cannot be undone.
Some promises are paid for in blood. She'd pay every last drop to keep hers.