Ardyn discovers Prompto really enjoys being high during sex. Cue him casually offering (or secretly dosing, if you're feeling evil) Prompto various substances and taking full advantage of his dreamy/energetic/whatever state afterwards.

Bonus points for actual consent after Prompto's been tempted into it. Eternal admiration for teasing and bondage. (And extra bonus points for needles, because I am an utter sinner)