I’m just making this up as I go along, really, so this may mutate without warning.
Much like Health, you have a Sanity track. You can just note Sanity next to your Health track. Much like Health, you can have Lethal and Non-Lethal damage; we’ll keep those terms just so we’re consistent, even though you can’t die from going insane – at least not directly. You also have Penalty blocks and again (to stay consistent) it’s every 4th block: 5, 9, 13, and so on.
For physical damage you consult Health and Will; for Sanity damage you work with Will and Fate.
Taking Sanity Damage (Loss)
When you confront a situation that can cause Sanity Loss, you need to make a Will Check vs. the difficulty of the situation. This Will Check is not modified by your Sanity Injuries. Most ‘mundane’ situations are between difficulty 3 and 9. Unnatural horrors can be worse – much worse. If you make the roll, nothing happens, but if you fail then you take a Sanity Loss equal to the difficulty of the check minus your Willpower score – and if that’s 0 or less, you take 1 point of Sanity Loss.
Example: You walk into your kitchen to find a severed head in your sink, dangling from the faucet shoved through its eye socket. This is pretty gruesome, so you need to make a Will(7) check. You have a Will of 8 so you get to roll 2d+2. You only roll a 6, though, so you lose Sanity. Since 7 minus 8 is negative 1, you only take 1 point of non-lethal Sanity Loss.
Non-Lethal (temporary): This indicates you’re momentarily aghast and distraught. If you take Non-lethal sanity loss you’re -1d for your next action if it’s within a minute of the loss. After that you’re back on-balance, since the need to do something else (translate a rune, dodge a claw) brings your focus back.
Lethal (‘permanent’): Lethal damage indicates a more profound trauma, something that will occasionally prevent you from sleeping or cause you to cringe when you’re startled by a particular reflection. After taking a Lethal loss you lose your next action (as if you were stunned) and are -1d for your next action if it’s within a hour of the loss.
Note: Rolling for Sanity Loss does not count as an action here. That’s passive, not active.
Non-Lethal: You gain back 1 Sanity per modified Will dice per day. You add 1 die for resting and another die for having comfortable surroundings. Relaxing in a hospital means you gain +2d. Relaxing in a storm gutter is just +1d.
Lethal: You can only regain Lethal Sanity Loss from prolonged treatment or for great successes, e.g., defeating a force of Cosmic Evil. I won’t get into the treatment systems right now. The “we won” method happens when I hand out Experience, and will typically be a simple roll of 1d3.
Injuries work a little differently with Sanity. First up: when you take an Injury (incur a die penalty) you must make an modified Will roll at a difficulty of however much Sanity you’ve lost.. If you make the roll nothing happens, otherwise one of your current non-lethal losses (including any you just took) becomes Lethal.
Example: Continuing with the head-in-the-sink scenario, it turns out you were already down four Sanity when you saw it The additional loss pushes you over to an Injury, so you must make a Willpower roll, difficulty of 5 (existing 4 damage + the one you just took). You roll a 4, and that 5th hit you took becomes Lethal.
Note – Cosmic Horrors – creatures that exude extradimensional energies or are profoundly alien – cause the human brain additional trauma. When you take an Injury because of witnessing a Cosmic Horror, the roll is made at an additional +2 difficulty. Had there been a Shoggoth in the sink instead of a severed head, the Willpower roll difficulty for Lethal Sanity Loss would’ve been 7, not 5. This penalty does not apply to the derangement roll mentioned below.
Derangement Roll: In addition, if you fail an unmodified Will roll against the same difficulty after taking a Lethal Loss, you gain a temporary derangement – a phobia, an obsession, a psychotic break, something. You and the GM should work that one out. This will last for 3d6 minutes…
…unless you then fail an unmodified Fate roll at a difficulty of the most recent Loss + 1. Then it’s permanent until treatment can cure you. Please note that (for playability purposes), permanent derangements tend to be less overwhelming than temporary ones, more of a background hum than a overwhelming fixation.
Example, continued: After a quick talk with the GM, you decide the sink-head scenario has given you an obsession with drains, wells, and other holes, since your irrational mind believes something must have come up out of the plumbing. You flee the kitchen and run upstairs, frantically taking a plunger to the toilet, the shower drain, and nearby wall outlets, trying to extract the monsters via suction. You scurry around this way for 3d6 minutes, then you then roll your Fate at a difficulty of 2 (1 + the damage you took, which was 1). In the unlikely event you fail this roll, the obsession becomes permanent. While you won’t be constantly taking a plunger to everything, you can’t pass a well without throwing a rock in and hearing it hit water (and you’re usually sure it doesn’t, it just kept falling, does anyone have a match? No, an electric torch, a match will just go out…). Consider this a 1-level shift Personality Trait – see section 2.17 of the rules for more detail.
Madness as Armor – Much like Physical Injuries, Sanity Injuries have a positive side-effect, somewhat. You subtract your penalty from the damage taken. So if you’re down 1d but and you’d take a 3 point loss, you only take 2 points of loss. As usual, the minimum loss is still one point.
Example: You’ve had a rough time of it and are down 9 points of Sanity, bringing you to -2d. A Shoggoth squeezes its bulk up through the toilet, sending porcelain and protoplasmic flesh flying everywhere. This is an uncomfortable sight, and you must make a Will(13) roll. Your Willpower is 9 and so your roll is 3d+0, but you’re down 2d so you just have the 1 die to roll. You fail outright. The GM would then slap you with 4 points of damage (13 – 9), but since you’re already down 2d you only take 2 points of damage.
Much like a KO Threshold, you have a Madness Threshold of your Willpower score x2. Once you hit that threshold, you use your unmodified Fate dice instead of your Willpower dice to see if you take a Loss. If you ever take any more Sanity Loss you are ‘permanently’ insane. Any existing derangements become magnified back to temporary levels, and you’re otherwise a gibbering wreck. Several months in an institution might – might – help.
Putting All This Mess Together In One Big Example of a Really Bad Night for Our Hero
Meet Pat Schamltz. Pat has a Willpower of 8 and a Fate of 4, so Pat has 2d+2 Willpower dice and 1d+1 Fate dice. Pat has a Madness Threshold of 16 (Willpower of 8 × 2).
Pat goes to a railyard to investigate a disappearance. Pat goes alone because Pat is rather stupid. While puttering about with a lantern Pat stumbles across a body on the tracks, head crushed like a melon by a train car and the body torn open, emptied of viscera. This is not pleasing to Pat’s eyes and thus Pat must make a Sanity check. The GM decides this is a pretty awful sight and thus gives it a difficulty of 7. Pat must roll a Willpower roll against a difficulty of 7. Pat rolls a 5 and a 4, and with that +2 has a total of 11 and thus maintains a grip.
A sound brings Pat’s attention to the open door of a railcar. It’s the sound of someone sucking pudding through a straw – and then half a body flies out from the interior darkness, skidding across the gravel to land at Pat’s feet. Pat steps back, but in the lamp light still sees the jaw working and hears the corpse rasp “still alive still alive still alive”…
The GM decides this is something of a bummer and makes Pat roll another Sanity check, this time with a difficulty of 9. Pat fails this one, only getting a 7 total on the Willpower roll. Pat loses 2 Sanity and thus has a -1d penalty for the next action, presuming it happens within a minute.
While Pat stands there in terror, spade-tipped tentacles pour from the darkness within the rail car, dragging a Dark Young out into the doorway frame. Oh rapture. Pat gets to make another Sanity check, this time with a difficulty of 11. Pat only rolls a 9, though, and so loses 3 more Sanity – 11 minus Pat’s willpower of 8. This pushes Pat into Injury territory. Go Pat!
Pat’s down 6 Sanity now, so first must make a modified Willpower(6) roll. That 1d penalty give Pat a Willpower Dice Pool of 1d+2. Like a boss, Pat rolls a 1, so the first non-lethal Sanity Loss becomes Lethal (permanent). So now Pat’s at risk for a Derangement.
Pat then rolls his regular Willpower dice, 2d+2, against the same difficulty (6). Pat rolls a total of 5 and so gets himself a temporary derangement. A quick discussion means that Pat now has a phobia about portals (archways, doors, etc.). He’s going to be have irrationally about that for 3d6 minutes. Is this derangement permanent?
Pat finally makes an unmodified Fate(3) Roll – the 3 comes from 1 + the recent 2 point loss. Pat rolls a total of 5, so the Derangement isn’t permanent, but a roll of 10 on 3d6 means that Pat wants nothing to do with openings or enterances for ten minutes. Because Pat took a Lethal Loss, Pat’s stunned and will be -1d for the next action.