Sanity checks are made when a character views a monster that has
negative Charisma and the character's Wisdom is below 0.  This is
usually the result of failing a Horror check, or a combination of
other possibilities (equipment that provides negative statistics,
curses, etc.).  Sanity checks compare the character's level against
the monster's negative Charisma and level.  Failure causes the
character's mind to snap and gain a random mental illness.  Mental
illnesses vary greatly, an individual's reaction can range from
berserk rage to screaming terror to a complete withdrawal from reality.
Sanity checks are also made for terrified characters who have very
high Wimpy and flee from a monster.