Moonlight (2007 TV Series) - Season 1 - Episode 16: "Sonata"

Beth encounters Emma Monaghan, a vampire who transformed the man she loved for eternal companionship. To her astonishment, Emma and her husband Jackson have remained together for 150 years. After Emma kills Dominic, a basketball player linked to Josef and other vampires, she is imprisoned. She threatens to reveal all vampires in Los Angeles unless Mick orchestrates her escape. Though Mick and the vampires assist her, they ultimately must eliminate Emma and Jackson for treason after her betrayal.