Night Reigns

Night Reigns is a paranormal romance by the American writer Dianne Duvall, published in 2011. It belongs to her Immortal Guardians series, in which a small number of long-lived warriors protect human beings from vampires that have lost control of themselves.

Marcus Graden has been an immortal protector for eight hundred years, and the life he has built for himself is deliberately solitary. He has no wish for company, and no interest in working with anyone, which makes the intervention of a stranger in the middle of a fight both useful and unwelcome.

The stranger is Ami, who is guarded and watchful and who turns out to be extremely good at covering somebody else's back. Marcus finds her competent, intelligent and entirely too close, and the novel follows the two of them as the arrangement becomes a partnership and then something that disturbs the isolation he has arranged around himself. Ami has secrets of her own, and the largest of them is the reason she appears not to be able to die.