Sanity as Skill / Sanity Track Combination
Starting San: The higher of Wis or Cha
Maximum San: 20 - (Knowledge: Mythos)
When confronted with something horrible, make a Sanity check (1d20 + Current San) versus DC 20. If successful, usually no ill effects. If failed, move a number of levels down the sanity track indicated by the situation (1 level for a gross-out, 1d2 for a minor shock, 1d6+1 for something pretty horrifying, 1d10 or more for mind-blasting horror). If the amount rolled for sanity loss is greater than five, then the amount over five is reapplied to the sanity track as a persistent condition. For example, an investigator sees a betentacled thing devour a friendly NPC and fails their San check, so the GM rolls sanity loss against the character and comes up with 8. The first 5 points of sanity loss cause the investigator to be Overcome, but there are still 3 points of sanity loss to account for. These are reapplied to the sanity track as -3 persistent steps, to Staggered. Even if the character avoids fainting or going temporarily insane, they will not be able to use swift actions to recover to any higher level than Staggered until they get some rest.
If the character is Overcome and then is also Overcome as a persistent condition, they have lost 2 points of current San in one go and must make a Sanity check to avoid indefinite insanity (see below).
Normal: No penalties.
1 Step (Disoriented): -1 to all Defenses, Attacks, Skill, and Ability Checks (including Sanity checks)
2 Steps (Dazed): -2 to all Defenses, Attacks, Skill, and Ability Checks (including Sanity checks)
3 Steps (Staggered): -5 to all Defenses, Attacks, Skill, and Ability Checks (including Sanity checks)
4 Steps (In Shock): -10 to all Defenses, Attacks, Skill, and Ability Checks (including Sanity checks)
5 Steps (Overcome): Faint or go into Temporary Insanity
If Staggered or In Shock, immediately make an additional DC 20 Sanity check (modified by the Sanity Track penalty). If failed, your current San goes down by 1 point.
If Overcome, your current San automatically goes down by 1 point and you immediately make a DC 10 Sanity check as if In Shock (-10). If successful, move up +1 step to In Shock; you are likely to gain partial or complete amnesia of the event as your mind goes into denial to protect itself. If the check has failed, you may choose to faint on the spot or go temporarily insane.
Faint: You are unconscious, as if you have fallen -5 steps on the condition track. You stay that way for at least 1d4 rounds. After that, you may be awakened by another character in the same or adjacent square using a standard action to shake, slap, or nudge you; if you succeed a DC 20 Endurance check (which you may attempt once per turn); or if you take damage. If left to recover on your own, you will awaken in 2d4 x 10 minutes.
Temporary Insanity: All penalties are removed, but your behavior is all crazygonuts. The effects of temporary insanity will probably stick with the OGL model.
Someone trained in the Psychoanalysis skill may spend a full-round action to make a Psychoanalysis check to bring a sanity-damaged or temporarily insane character back to their senses. The DC is 10 + the levels of Sanity loss the character has suffered. A successful check brings them back to the level of their highest persistent step (e.g., Staggered in the case mentioned earlier), or normal if they have no persistent steps; a failed check has no effect. This does not recover any lost Sanity points.
On each turn you are not temporarily insane, you may spend 1-3 swift actions to move up +1 level on the Sanity Track (going up +1 step for every swift action spent) up to the level of your highest persistent step. Persistent sanity effects remain until your character has had 10-15 minutes of quiet contemplation per step to recover their composure (or at least get a stiff drink).
If your current San goes down by 2 or more points during a single encounter, you must immediately make a DC 10 Sanity check (with any Sanity Track modifiers) or succumb to indefinite insanity. This generally removes your character from active play for quite some time, although they may recover and return on a future occasion.
By giving long-term treatment to a character who has lost Sanity, a character trained in the Psychoanalysis skill can help them restore some of their Sanity. For each month of treatment, make a DC 20 Psychoanalysis check to restore 1 point of current San, but you cannot bring them above their starting San or maximum San. This can also be done for a character who has been removed from play for indefinite insanity (but not permanent insanity). If the check is failed by 5+, the patient gets worse instead of better, losing 1 point of current San instead.
Your current San goes up by +1 every time you gain a level. This may increase it past starting San (although in practice this is rare).
Good points: This incorporates the downward spiral, players rolling the dice for their sanity checks, some granularity of effect, and consistency with the SWSE mechanics.
Bad points: Counting track steps, particularly for persistent steps, may be a bit clunky. Also, instead of the slow, inevitable trickle of CoC-style, this system tends to lose fairly big whacks of Sanity at a time. (1 point of a 1-20 scale is like 5 points of a 1-100 scale.)
What do you think, sirs? I'm open to questions, comments, criticisms, or suggestions!