Sanity

In CoC your sanity is as important as your Hit Points -- perhaps more important. As HP can be healed over time, your sanity erodes and can only be gained back slowly and through difficult means. Like XP in DnD, sanity is granted as a bonus for good work - vanquishing evil gives you the sense that your struggle against the unknown can be won and with it piece of mind. Your sanity can also increase with Psychiatric care and drugs or by increasing your POW. (there is a third way, but we'll talk about that later)

Your sanity begins at 5x your Power. As the game progresses, different things can cause this to erode. Unseemly sights (though some professions are immune to the sight of blood or corpses) , ill-gotten knowledge, and shocking events all take their toll.

There is a skill called Cthulhu Mythos, which represents your knowledge of the greater mysteries in the universe. All characters begin with 00 Mythos. These mysteries are so terrible they have an effect on your sanity. To represent this, your total sanity can never be greater than 99 minus your Cthulhu Mythos skill. (For example: if you have a CM skill of 35, your sanity cannot be greater than 65)

When something horrible happens you will be asked to make a Sanity Check. You must roll a % roll under your current Sanity or you have failed. When you fail, you lose sanity - usually a random amount based on how horrifying the event. If you lose 5 sanity points from a single roll, you go temporarily insane for 1d10+4 rounds. This can be anything from a fainting spell to homicidal mania.

If you lose 1/5 of your current sanity in a game hour, you go indefinitely insane. This insanity lasts 1d6 months. Some insanity can be functional, like amnesia or claustrophobia, but it should be debilitating. Some investigators when they have lost this much sanity and are dangerously low will check themselves into a sanatorium for treatment.

If you reach Zero sanity points. Game over. You are permanently insane and non-functioning. A blathering idiot who needs a straight-jacket. This is the death of the mind and it is as dangerous as the death of the body.

In DnD, when you have 1 hp left, you act like it: a roleplayed cough, frail movements, or pain. You may also hang back if the action is in full swing -- because one wrong move means you're dead. Sanity in CoC should be treated no differently. People with low sanity are on the verge with frayed nerves. They may also protect themselves from having to bear more shock by letting their more stable compatriots do the dirty work.