It only started as a small sacrafice
The horrors of the Cthulhu Mythos threaten the mind as much as the body. The psychic shock from encountering the alien terrors of the Mythos is one of the primary dangers that characters in the game will endure. Investigating mysteries in a Call of Cthulhu game take a dreadful toll on the sanity of the investigators, and eventually lead to madness. Whenever you encounter the horrors of the Mythos, or come across something mundane yet horrific (such as stumbling across your best friend’s mutilated corpse) you make a percentile roll against your current Sanity score. If you roll over your current Sanity, you lose a greater amount of Sanity points. If you roll under, you will lose less, or none. The Sanity loss is generally described for an event as something like “0/1D6” or “2/1D10.” The number before the slash mark tells you how much Sanity your character loses if he rolls under his current SAN score; the number after the slash is how much your character loses if you roll over your current SAN.
When confronted with sanity-blasting events, your Keeper will ask you to roll the percentile dice
and then let you know how much SAN you lose depending on if you succeed or fail. Unfortunately, regaining lost Sanity is a long, arduous process. You may have to check your character into an asylum, or seek other forms of psychotherapy to get those points back. Generally, at the successful end of each scenario you should get a few points back as a reward. Additionally, when
you raise a skill above 90% through experience, you get a few points added to your Sanity as well.
As your SAN score slips lower, your character becomes less and less stable and his ability to function decreases. Full rules for Sanity are not included here, but your keeper will let you know the effects of this degradation when you play the game.