Good insanity game mechanic

[Kurik Thurgen]

I've been planning to run a CoC themed campaign and find the d20 rules quite good for most aspects of the game, I just need to have a good system for player and npc sanity. I'm thinking that players start off 100% sane but when exposed to alien monsters or disturbing events they would need to make a will save or take Xd10 sanity damage unless they spend time resting their minds. I also think that sort of DR for sanity could be acquired with feats. The campaign is going to be lower magic at the beginning with players needing to access occult tomes to use spells and having to roll a d20 each time they cast with a roll of a natural 1 or a roll under double the spell's level requiring a sanity save. Any other suggestions?

[Silent Saturn]

There is a mechanic for various mental illnesses described in the Game Master's Guide. Basically it's a number of mental illnesses and the symptoms of each, plus a Will save DC. If your Int, Wis, or Cha reaches zero through effects that damage them, you wake up the next day with a mental illness of the GM's choosing. Each day you make a Will save. If you make it, subtract the amount you beat it by from the DC for your next save. When the DC reaches zero, you've recovered.

I'd look into just having abberations deal Int or Wis damage, and then applying these rules for when a player's score in one hits zero. Spellcasters will have to defend themselves along a line they're not used to defending themselves, and martial types who dumped those stats will find that their 30 Ac and 100 HP don't make them as invulnerable as they had expected.

[Kelsey MacAilbert]

3.5 Unearthed Arcana has rules for this precise thing, as does Heroes of Horror (In fact, I do believe the two sets of rules are the same, and that the one was ported into the other). They are very good rules, do precisely what you seem to need, and, being UA, are available free:

http://www.dandwiki.com/wiki/UA:Sanity

[Azothath]

{yes... this is best place for this...}

hmmm... could refer to Sanity & Madness and Types on AoN and they refer to GameMastery Guide circa p250. The chart omits dementia, senility, alzheimers, autism.... I suspect the list is a bit short, see List of Mental Disorders on wikipedia (spoiler below). PF/DnD game system doesn't have the accuracy to distinguish between psychological and neurological based afflictions.

List of (220) Mental Disorders:

Adjustment disorder
Agoraphobia
Alcohol use disorder
Alcoholic hallucinosis
Alice in Wonderland syndrome
Alzheimer's disease
Amnestic disorder (Amnesia)
Amphetamine dependence
Anorexia nervosa
Anterograde amnesia
Antisocial personality disorder
Anxiety disorder
Asperger syndrome
Attention deficit hyperactivity disorder (Adhd)
Autism spectrum disorder
Autophagia
Avoidant personality disorder
Avoidant/restrictive food intake disorder
Atypical depression
Binge eating disorder
Bipolar disorder
Body dysmorphic disorder
Borderline intellectual functioning
Borderline personality disorder
Brief psychotic disorder
Bulimia nervosa
Caffeine-induced anxiety disorder
Caffeine-induced sleep disorder
Caffeine dependence
Caffeine withdrawal
Cannabis dependence
Capgras delusion
Catalepsy (Narcolepsy)
Catatonia
Catatonic schizophrenia
Childhood amnesia
Childhood disintegrative disorder
Childhood onset fluency disorder
Child neglect
Circadian rhythm sleep disorder
Claustrophobia
Cocaine dependence
Cocaine intoxication
Cognitive disorder
Communication disorder
Conduct disorder
Conversion disorder
Cotard delusion
Cyclothymic disorder
Delirium
Delusional disorder
Delusional parasitosis (Ekbom's syndrome)
Dementia
Dependent personality disorder
Depersonalization disorder
Depression
Depressive personality disorder
Dermotillomania
Developmental coordination disorder
Diogenes syndrome
Disinhibited social engagement disorder
Disorder of written expression
Dispareunia
Disruptive mood dysregulation disorder
Dissociative amnesia (Psychogenic amnesia)
Dissociative identity disorder
Down syndrome
Drug withdrawal
Dyscalculia
Dyslexia
Encopresis
Enuresis
Erectile disorder
Erotomania
Excoriation disorder
Exhibitionistic disorder
Expressive language disorder
Factitious disorder
Female sexual arousal/interest disorder
Folie à deux (Shared psychotic disorder)
Fregoli delusion
Frotteuristic Disorder
Fugue state
Gambling disorder
Ganser syndrome
General adaptation syndrome
Generalized anxiety disorder
Grandiose delusions
Hallucinogen persisting perception disorder
Haltlose personality disorder
Histrionic personality disorder
Hoarding disorder
Huntington's disease
Hyperkinetic syndrome
Hypersomnia
Hypoactive sexual desire disorder
Hypochondriasis
Hypomanic episode
Hysteria
Illness anxiety disorder (Hypochondriasis)
Impulse control disorder
Inhalant use disorder
Insomnia
Intellectual development disorder
Intermittent explosive disorder
Internet gaming disorder
Kleptomania
Korsakoff's syndrome
Lacunar amnesia
Language disorder
Learning disorder
Major depressive disorder
Major depressive episode
Maladaptive daydreaming
Male erectile disorder
Malingering
Mania
Melancholia
Minor depressive disorder
Misophonia
Mood disorder
Morbid jealousy
Munchausen syndrome
Munchausen by proxy
Mixed episode
Narcissistic personality disorder
Narcolepsy
Neurodevelopmental disorder
Nicotine dependence
Nicotine withdrawal
Night eating syndrome
Nightmare disorder
Non-suicidal self-injury
Obsessive–compulsive disorder
Obsessive–compulsive personality disorder
Obsessive love disorder
Oneirophrenia
Onychophagia
Opioid dependence
Opioid use disorder
Oppositional defiant disorder
Orthorexia nervosa
Other Specified Dissociative Disorder
Other specified feeding or eating disorder
Pain disorder
Panic disorder
Paranoid personality disorder
Parasomnia
Parkinson's disease
Partialism
Passive–aggressive personality disorder
Pathological gambling
Pedophilic disorder
Perfectionism
Persecutory delusion
Persistent complex bereavement disorder (Complicated grief disorder)
Persistent depressive disorder (Dysthymia)
Personality disorder
Pervasive developmental disorder
Pervasive developmental disorder not otherwise specified
Phencyclidine-related disorder
Phobic disorder (phobia)
Phonological disorder (Speech disorder)
Physical abuse
Pica (disorder)
Polysubstance-related disorder
Postpartum depression
Post-traumatic embitterment disorder
Post-traumatic stress disorder
Premature ejaculation
Premenstrual dysphoric disorder
Primary hypersomnia
Pseudologia fantastica (Pathological lying)
Psychogenic amnesia
Psychotic disorder (Psychosis)
Pyromania
Reactive attachment disorder
Reading disorder
Recurrent brief depression
Relational disorder
REM sleep behavior disorder
Residual schizophrenia
Restless legs syndrome
Retrograde amnesia
Rett syndrome
Rumination syndrome
Roberts syndrome
Sadistic personality disorder
Schizoaffective disorder
Schizoid personality disorder
Schizophrenia
Schizophreniform disorder
Schizotypal personality disorder
Seasonal affective disorder
Sedative-, hypnotic-, or anxiolytic-related disorder
Selective mutism
Self-defeating personality disorder
Separation anxiety disorder
Shared psychotic disorder
Sleep disorder
Sleep terror disorder
Sleep paralysis
Sleepwalking disorder
Social anxiety disorder (Social phobia)
Sociopathy
Somatic symptom disorder
Somatization disorder
Stendhal syndrome
Stereotypic movement disorder
Stockholm syndrome
Substance-related disorder
Tardive dyskinesia
Tourette syndrome
Transient global amnesia
Transvestic disorder
Trichotillomania
Truman syndrome
Undifferentiated somatoform disorder (Somatic symptom disorder)
Vaginismus
Voyeuristic disorder
===end===
I didn't qualify the list, so you'll have to deal with items like 'Internet gaming disorder'

Secondary mental disorders increase the DC +5 rather than like afflictions(poisons) at +2. I'd move it to the lesser penalty as that also should affect the check to cure it.

In Call of Cthulhu, the rules are more focused about the mundane, skill checks (heal checks) and regaining Sanity loss. They don't have the breadth or inundation of magic that exists in PF/DnD. One heal spell would negate the tenacious horror aspect of PC existence in CoC.

Sanity from Unearthed Arcana {duplicate} DnD 3.5 which is OGL {thanks to Homebrew forum}
and
Call of Cthulhu v7 Quick Start Guide pdf by Chaosium.

[Ryze Kuja]

Sanity Rules might be what you're looking for.

[TxSam88]

check out Carrion crown AP, book 4.

[Azothath]

TxSam88 wrote:

check out Carrion crown AP, book 4.

very simple rules, in sidebar. PZO9046 pg29 c2011

Insanity and the PCs {paraphrased}:

(starting) Sanity=Lvl+WIS mod
with each 'shocking' encounter Will 15 or some sanity loss. Sanity loss varies from about 1d4 to 1d6 for seeing a mythos creature, but up to 2d6 for horrifying scenes, it's rather random.
Defeating the crtr allows half the sanity loss to be regained (LoL).

When San=0 character afflicted with insanity from GMstryGd p250. {No indication that sanity can't go worse(negative) or stops at this point.}
Such insanity can only be cured with greater restoration, heal, limited wish, miracle, or wish, or as described under each individual form of insanity’s entry, which also restores a PC’s Sanity points to full.
GM Option: when PC goes insane Wis 15 Chk to gain insight into event (similar effect to successful Knowledge check).

rules with later publication date usually supplant earlier rules, especially if they appear in the core product line.
Pathfinder Roleplaying Game Horror Adventures © 2016 (d20 link in Ryze Kuja post) updates the above rules using IWC and the midpoint of Max Sanity as the threshold (allowing lower sanity scores for more insanity). It also scales with CR making it easier on the home GM. The rules do cause a minor sanity loss with each new event (unless you're a newbie, "The More you Know - the More you Go!").

[Tim Emrick]

If you want the feel of CoC's sanity rules but d20-based rules for everything else, there's always Call of Cthulhu d20. It's designed with 20th-century characters and technology in mind (just like original CoC), so the only access to supernatural power for PCs is spells and the occasional magic item, both of which have a way of quickly eroding one's sanity (again, much like original CoC).

[kadance]

Way late to the party, but here's my take on it for my Strange Aeons campaign: LINK

[Goth Guru]

In my version, Mythos is the mind adjusting to the natural laws and "logic" of a distinctly dissimilar set of planes of existence.
0-They view mythos creatures as monsters with the ability to magically cause insanity.
1-Eccentric seeming
5-Confusing the mundane with the Mythos. May show physical alterations.
9-Becoming a mythos creature in mind and body.