In a mixing bowl, combine the ranch dressing, Cajun seasoning, and a pinch of cayenne pepper powder.
Stir well, then refrigerate for 1 to 2 hours prior to using (to allow the flavors to meld).
Use your judgment on the amount of cayenne pepper used. Its only purpose here is as an additional heat source to amp up the spiciness. You can omit it all together, or increase it for something even spicier.