As it happens, I’ve been writing a story involving poker. In that story, the cards are marked with ink that can only be seen under a blacklight. This is done to prevent people cheating by having a literal ace up their sleeve.
But you could do something else with the invisible ink. Maybe, at a random point during the came, the lights go out and the black light comes on, revealing that certain cards have something written on them. Maybe its a sexual act they have to perform, or a person they have to perform it with. Or maybe it’s the chips that are marked.