Master of Seduction (Merlin's Legacy 1) by Angela Knight
All her life, Sheriff's Deputy Rachel Kent has dreamed of becoming one of the immortal Magekind witches who protect humanity from itself. But first she must prove herself to the handsome vampire whose job is to decide whether she'll become a danger to those she's supposed to save.
Nathaniel Allard is a Court Seducer who has been sent to trigger Rachel's witchy transformation by making love to her three times. The problem is, gaining such incredible powers may drive Rachel insane and force Nathaniel to kill her. Otherwise she may kill him -- and anyone else who gets in her way.
Nate vows he'll only agree to complete Rachel's transformation if she proves she can be trusted with the powers she'll gain. But as he tests her -- and makes love to her with every test she passes -- mutual lust becomes something more. Will love be enough to save Rachel's sanity?