In Pathfinder Bestiary 4 (so says the website) The Great Old Ones have a defensive ability listed as "Insanity" with a save DC. How does that work? (How do I apply that to a game?)
From the Great Old One subtype:
Insanity (Ex) Any creature that attempts to interact directly with a Great Old One's thoughts (such as via detect thoughts or telepathy) must succeed at a Will save or be driven permanently insane. The save DC is equal to 10 + 1/2 the Great Old One's Hit Dice + the Great Old One's Charisma modifier. This duplicates the effect of the insanity spell, or the insanity rules (in which case the save DC is also the ongoing insanity DC). A Great Old One using its telepathy to communicate doesn't activate this ability unless it spends a standard action to focus its mind on one opponent. This is a mind-affecting effect.