Sanity rule

as in the Cthulhu Mythos,
Sanity is the natural mental state of ordinary life. Normal mental balance is endangered when characters confront horrors, entities, or activities that are shocking, unnatural, and bewildering. Such encounters cause a character to lose points from his Sanity score, which in turn risks temporary, indefinite, or permanent insanity. Mental stability and lost Sanity points can be restored, up to a point, but psychological scars may remain.

Sanity points: measure the stability of a character’s mind
A character’s starting Sanity equals his Wisdom score multiplied by 5
A character’s current Sanity score fluctuates almost as often as (and sometimes much more often than) his hit points
Sanity Check
When a character encounters a gruesome, unnatural, or supernatural situation, the DM may require the player to make a Sanity check using percentile dice (d%). The check succeeds if the result is equal to or less than the character’s current Sanity. On a successful check, the character either loses no Sanity points or loses only a minimal amount.
A character’s current Sanity is also at risk when the character reads certain books, learns certain types of spells, and attempts to cast them. These Sanity losses are usually automatic (no Sanity check is allowed)
Insanity
Losing more than a few Sanity points may cause a character to go insane, as described below. If a character’s Sanity score drops to 0 or lower, she begins the quick slide into permanent insanity

Sanity Resistance - This number is the amount of Sanity loss a character can ignore when he encounters a creature that requires a Sanity check or when he casts a spell that has sanity cost.
Equals to player's character level.

Sanity Loss
- Encountering the Unimaginable
- Severe Shocks
- Casting Spells - Magic relies on the physics of the true universe. By casting spells, characters visualize the unimaginable, warping their minds to follow alien ways of thought. These visualizations wound the mind. Although spellcasters expose themselves to such traumas voluntarily, they are shocks all the same
- Sanity affecting magic fear effects, illusions (some), contact other plane, commune, summoning, bestow curse
- Reading forbidden tomes - The burning power of a greater reality seizes the soul. Whether we try to retreat from the experience or hunger greedily for more, it destroys our confidence in what we once believed, opening us up to the all-encompassing truths of dark deities.

Character insanity is induced by a swift succession of shocking experiences or ghastly revelations, events usually connected with dark gods, creatures from the Outer Planes, or powerful spellcasting.
Horrifying encounters can result in one of three states of mental unbalance: temporary, indefinite, and permanent insanity. The first two, temporary insanity and indefinite insanity, can be cured. The third, permanent insanity, results when a character’s Sanity points are reduced to -10 or lower. This condition cannot be cured.

Temporary Insanity
Whenever a character loses Sanity points equal to one-half her Wisdom score from a single episode of Sanity loss, she has experienced enough of a shock that the DM must ask for a Sanity check. If the check fails, the character realizes the full significance of what she saw or experienced and goes temporarily insane. If the check succeeds, the character does not go insane, but she may not clearly remember what she experienced (a trick the mind plays to protect itself).
Temporary insanity might last for a few minutes or a few days. Perhaps the character acquires a phobia or fetish befitting the situation, faints, becomes hysterical, or suffers nervous twitches, but she can still respond rationally enough to run away or hide from a threat.