The Deep Vault
Sanity will be used in this campaign. I am taking most of the rules about sanity from here with slightly fewer rules. Here are the things you need to know to begin.
Your base sanity is your wisdom score times five.
(Wis x 5).
PC John has a wisdom score of 16, therefore his base Sanity (and starting sanity points) is 80.
Certain things trigger a Sanity Check, similar to a Reflex or Will save.
A Sanity Check is made by rolling percentage dice (the two ten-sided dice).
If the result is equal to or lower than your current sanity points, you succeed.
If it is not, you fail.
Ex.) PC John encounters an aberration. Normally, he would shrug it off, but it happens to be eating the last bit of a female torso, belonging to a pretty waitress he had encountered earlier that day. John recoils in disgust and surprise, and he must make a Sanity Check.
Johns current Sanity is 80. He rolls the percentage dice, and the result is 43. That’s lower than his current Sanity, so he keeps his head about him, and proceeds to slay the monster.
Later, John walks into his room at the inn, only to find a package on his desk, addressed to him. He opens the package and finds the head of a dog, bloody and eyeless and sopping with fresh maggots. This is quite a deviation from his normal mail, so he makes a Sanity check.
He rolls the dice and the result is 87, which is higher than his current Sanity. He fails the save, and takes 1d6 Sanity loss. The result is 4, so his total Sanity is now at 76.
Next, a character who takes, from one event, half his Wisdom score or more in Sanity loss, they may undergo a bout of temporary insanity.
Ex.) John returns to the inn and again, finds a package addressed to him on his desk.
His current Sanity is 76. His Wisdom score is 16. He reluctantly opens the package, expecting to find another dog’s head, only this time what he finds belongs to no animal; it is the decapitated head of his mother, with the eyes gouged out and replaced with the dog’s eyes.
He makes a Sanity check and fails, taking 2d12 Sanity loss. He winds up losing 9 Sanity, which is over half his Wisdom score. He recoils in horror and begins to babble incoherently, curling up in a ball on the floor for the next 2d10 minutes.
Some spells and effects will affect Sanity rather than the normal effect. (Bestow curse, for example, as mentioned in the link above, can force a Sanity check instead of ability drain).
For example, a PC under the effect of a Curse of The Putrid Husk may need to make a Sanity check as well, as they believe their insides are falling out.
Lastly and perhaps most importantly, a character committing terrible things may begin to lose Sanity. I realize sometimes playing an evil character is fun, but I want this campaign to feel realistic, therefore I am considering using Sanity loss in the following way as well.
Ex.) PC Gregory Steiner wants to rob some poor person in an alleyway. He does so, pilfering a handful of precious gems and a satchel of gold. This act, though unlawful and morally questionable, would not garner any checks.
However, now the robbed aristocrat begins to get up, screaming for help about being mugged. PC Gregory Steiner proceeds to cut out the tongue of the aristocrat. The guttural sounds of blood-choked gasps and half moaned screams, the sight of eyes going bloodshot from the pain, and the general strangeness of the entire event happening in the middle of town at 10 am would cause PC Gregory Steiner to make a Sanity check, as well as any PC that did nothing to stop it and who considers themselves a non-evil person.
Cognitive Dissonance causes Sanity loss, in this campaign just as in real life.