The Sweetest of Words (Have the Bitterest Taste)
“Ah, yeah, Desiree, I told you I was meeting someone. Well, that someone is Derek. My boyfriend. We’re totally in love.” His heart was racing and Derek was holding him so tight it was difficult to turn enough to face the young woman. What he did see of her had his breath catching on fishhooks in his throat. She was normally a relatively pretty girl, with cute round cheeks and large dark eyes, but in that moment she looked…terrifying. Her cheeks seemed gaunt, her eyes glowing like they were little windows peeking into a deep pit of raging flame.
(Or: Five or so years after the show. Stiles is in college, and finds himself getting stalked by a succubus. Derek's determined that the best way to thwart her is to prove that he and Stiles are madly in love. It's not really as much of an act as either seems to think.)