If the DM declares that your game is using sanity rules, then you immediately need to calculate how many sanity points you have.
There is no distinction between current and maximum sanity, the equation merely determines how much sanity you start with.
50 Base Sanity + (INT x 3) + (WIS x 3) + (CHA x 3)
There is no minimum +1 for these calculations, negative stats are included
Sanity Tests
In games that use Sanity Rules, there will be many times where you witness something that tests the limits of your sanity. Once per combat or once per 10 minutes out of combat you can be called to make a sanity roll. When this happens, roll a d20 + your will bonus. Usually you would add a mental stat, but not in this case.
Failing a sanity roll will result in you taking sanity damage and the DM will force you to take an involuntary action on your next turn. This type of damage doesn’t effect your hit points, it permanently lower your maximum sanity points. Involuntary Actions can include forcibly unequipping what you’re holding, screaming or making a noise, running away as fast as you can, etc.
If you take at least 5 sanity damage at once, then you need to roll an investigation check. If you roll above a 10, you will experience a meltdown.
The average damage dice used for sanity loss is a d6. However it can be higher depending on how bad someone fails or the source of the test itself.
Crit rules apply to sanity loss.
This is optional, but you can choose to give someone a phobia instead of dealing sanity damage to them. A phobia will give someone the fear condition whenever they see the source related to the sanity test. This automatic fear condition can be mitigated with social rolls and abilities from other characters.
(make sure not to just give them a phobia based on the main horror of your game, that would be really lame if they just run in fear anytime they interact with your horror)
Meltdowns
After failing a sanity check that deals at least five damage, managing to understand the horror that tests your sanity causes you to have a meltdown. The DM can roll a 1d10 or select 1 of the following options:
1 - For 1d10 minutes/rounds you have no memory of events that have taken place since your last long rest. It seems to them that one moment ago they were falling asleep and now they’re in front of a monster.
2 - For 1d10 minutes/rounds you become blind, deaf, disoriented, or you lose the use of a limb temporarily. I recommend rolling a d4 to see which effect you get afflicted with.
3 - For 1d10 minutes/rounds you must use your action to attack the nearest character or creature. If you have effects that give you multiple attacks then you must use them.
4 - For 1d10 minutes/rounds you see everyone around you as an enemy that cannot be trusted, including your party members. You cannot accept help from anybody or attempt to help someone.
5 - For 1d10 minutes/rounds you see someone around you as someone else in your backstory. As the DM make sure this is someone important that will change how the player will act in this circumstance, either someone that the player would give their life for or a horrible person they still haven’t forgiven.
6 - For 1d10 minutes/rounds you instantly gain the unconscious condition.
7 - For 1d10 minutes/rounds you begin fleeing as fast as you can in the opposite direction without moving towards the source of the sanity test. You must use every dash effect at your disposal every round.
8 - For 1d10 minutes/rounds you gain the stunned condition due to involuntary laughter, screaming, crying, etc. You also cannot use dash effects while you are stunned like this.
9 - Gain a new phobia. A phobia will give someone the fear condition whenever they see the source related to the sanity test. This automatic fear condition can be mitigated with social rolls and abilities from other characters.
10 - Gain an obsession. This can be something like a compulsion for lying, washing oneself, being obsessed with something. Alongside being forced to perform an involuntary action, for 1d4 - 1 minutes/rounds after losing sanity you need to dedicate those turns to your obsession.
Consequences For Losing Sanity
These are looser suggestions for dealing with characters that have gotten low on sanity.
They can start seeing things that aren’t there
Their meltdowns start lasting longer
It’s harder for other characters to use social rolls and abilities on them
Negatives to their will bonus and other mental skill checks
They can gain a mental illness, the effects of which would be connected to sanity loss in some way.
(Do not misinterpret that mental illness always correlates to going crazy)
Be liberal with these, it won’t be fun if they get to lower sanity and you just say “Well you can’t be talked out of a mental breakdown anymore also mark down -10 to will.” These rules are meant to be harsh but still part of the fun game we all play.
Regaining Sanity
There are technically ways to regain sanity. If you have spent at least a couple days away from an environment that tests your sanity, you can attempt to increase your sanity the same way you can train to increase your HP. Note that it should be a really difficult combo roll and it’s unrealistic that you can just sit down to meditate and suddenly you’re a little less crazy then before. Regaining sanity will be a long enduring process.