When Cthulhu uses his nightmare spell-like ability on a creature with one or more ranks in a Craft or Perform skill, he also afflicts the creature with maddening dreams. In addition to the effect of nightmare, the target must succeed at a DC 40 Will save or contract a random insanity (Pathfinder RPG GameMastery Guide 250). This is a mind-affecting effect. A creature that already has an insanity is immune to this ability. The save DC is Charisma-based.