Eion by Alyse Anders

Eion is the second in command of the Qadrus, an elite squad of cyborg soldiers. It’s been lonely work, and with the war long over, Eion is now directionless. When he and his squad are needed for an experiment involving a group of human women, Eion agrees. What he doesn’t anticipate is the rush of lust he feels at the sight of his mate.
Meeting an alien race was the last hope Kaia had to find a cure for her terminally ill twin sister. She didn’t hesitate to board a spaceship, giving up everything in her life for the chance. What she hadn’t expected was to wake to discover that not only did they have a potential cure, but that she was the fated mate of an intense and attractive cyborg.
When the Black Guard kidnaps Kaia’s sister, she must put her life and her heart in Eion’s hands in the hopes that he can protect both.