Blood Oath by Belinda Boring

His life for hers.
No truer words were spoken.

Charged by the Fates to relive his darkest moments to date, Devlin Lockhart willingly sacrifices any future happiness to restore Darcy’s faith in him. Having watched over her from birth, Devlin never regretted the blood oath he made his sister, Elynor, over a hundred years earlier. But as he returns to some of the most painful memories he holds—ones that shroud the mystery of his conversion to becoming a vampire, will Devlin have the courage to stay true to his promises? Or will the savagery that marks his soul finally claim him?

Only time will tell.