You seek refuge from terrible truths by isolating parts of your mind. You gain an insanity of the GM’s choice (see Dread and Insanity in Chapter 4). Your GM should work with you to ensure its nature makes the game more fun for you and the group, and it should generally fall into the category of a first-degree insanity as described on page 79. As long as you have this insanity, you have advantage on Constitution saving throws to concentrate on spells and on all Wisdom saving throws. If your insanity is cured, you may either gain a different insanity or choose a new feat in place of this one.

First Degree: Neurotic Aftereffects After a brush with dread, or after recovering from a psychotic episode, it is unlikely that a person will completely return to normal. Instead, they will have quirks, nervous tics, and a somewhat shaky grasp of the world. If their insanity was instigated by reading terrible tomes, for instance, then other books may make the character nervous or even cause panic attacks. The character may hoard objects or avoid keeping anything. They may be constantly concerned about the possibility of insects crawling under their skin or constantly scratch at their exposed skin. The character might hear voices but (usually) knows that they are not real. Look for quirks to make your character fun, but not unplayable.

Sandy Petersen’s Cthulhu Mythos