Dark Series 3: Dark Gold by Christine Feehan
They were masters of the darkness, searching through eternity for a mistress of the light… Alexandria Houton will sacrifice anything — even her life — to protect her orphaned little brother. But when both encounter an unspeakable evil in the swirling San Francisco mists, Alex can only cry to heaven for their deliverance…
And out of the darkness swoops Aidan Savage, a golden being more powerful, more mysterious, than any other creature of the night.
The ageless Carpathian male snatches them from a hideous fate. But is Aidan Alex's salvation… or her sin? If she surrenders to Aidan's savage, unearthly seduction, and gives him the color, the light, the family he craves, will Alex truly save her brother? Or sacrifice more than her life?