When people perceive creatures and entities of unspeakable horror, this experience costs them some portion of their minds, since such creatures are intrinsically discomforting and repellent. We never lose awareness of their slimy, fetid, alien nature. In the most simple of terms, it affects our sanity.
In the Call of Cthulhu (CoC) game, Sanity is nearly as important as Hit Points in D&D. In CoC, the Sanity statistic functions like an ability score in some ways, but it has its own unique mechanics that represent the character’s descent from a stable and healthy mental state into confusion, dementia, and mental instability. As a character encounters monsters, witnesses horrible acts, masters forbidden knowledge, or casts spells, his Sanity score, and his corresponding ability to function as a normal person deteriorates. This gradual descent is balanced in part by the powers that characters gain each time they overcome a horrific foe or grow in skill and expertise, but even as those characters grow in power, they know or fear that an even greater peril lies ahead—the threat of becoming permanently insane.
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.
Insanity occurs if too many Sanity points are lost in too short a time. Insanity does not necessarily occur if Sanity points are low, but a lower Sanity score makes some forms of insanity more likely to occur after a character experiences an emotional shock. The character’s Sanity may be regained after a few minutes, recovered after a few months, or lost forever.
A character may regain Sanity points, and even increase her Sanity point maximum. However, increasing a character’s ranks in the Knowledge (forbidden lore) skill always lowers her maximum Sanity by an equal amount.
In the Call of Cthulhu Roleplaying Game, characters feel more vulnerable, for no matter how powerful they become, the dark gods are always greater. Religion is not only a source of comfort or succor, but also a dangerous enemy. Characters are suspicious, even paranoid, for a seemingly innocent commoner could secretly serve a cult. And yet, with such dark challenges come the opportunities for greater heroism.