Mythos Corruption – Optional Rules

The gateway opens to Utter Corruption. Art by Damir Damsa Omić

Call of Cthulhu encourages many different styles of play and formats, such as one-shot survival horror, short and deadly investigative modules, and long-form heroic campaigns to name just a few. The alternative Mythos Corruption framework we have presented may not mesh well with every game, but we would like to present some optional rules that could facilitate its incorporation at your table. 

Modified Mythos Resistance (Sanity) Costs

You may wish to compensate for the fact that your Investigators no longer suffer losses related to Mundane Horror by making a simple cost adjustment. Every check will result in the loss of at least one point of Mythos Resistance (Sanity). That means your Investigators will always experience some degree of Corruption when they have any contact with Mythos. We personally really enjoy this change. It means that your Investigators will always suffer the consequences of seeing minor entities like Ghouls, Deep Ones, as well as stumbling upon minor relics and unsettling art. 

Daily Vulnerability Check

Exhausted Investigators face relentless threats. Art by Gustave Doré

A daily Vulnerability check reflects the pressures of being an Investigator, which involves tracking cultists, witnessing violent events, disrupting their daily lives, and spending time away from family and friends. You can opt to begin every session with a Vulnerability check to determine an Investigator’s baseline risk.

Progressive Vulnerability Challenges – 3 Strikes

Instead of Vulnerability functioning as a binary state, you can allow for a series of Vulnerability checks with a progressive increase in difficulty from Regular to Hard to Extreme. This can reflect the difficulty in maintaining composure after exposure to multiple upsetting events. This can pair well with a daily Vulnerability check.

Along with the progressive challenges you could choose to add increasing consequences for failed checks. After the first failed check, the Investigator becomes Vulnerable, as reflected in the previous discussion. After the second failed check, the Investigator becomes Fragile, where they will experience Cosmic Horror after the loss of a single point of Mythos Resistance. After the third failed check, they will Break, at which point the player must choose between Fight, Flight, and Fetal Position options. This would increase player book-keeping but could add a lot of flavor to one-shot scenarios. 

Developing Resilience

As Investigators progress in a campaign, they may become resistant to the effects of recurrent violence, episodes of helplessness, and violations of self. Again, this requires additional book-keeping for players, so we avoid it, but we have enjoyed a similar mechanic in our Delta Green campaigns. 

Resilience may be developed when the Investigator has passed three Vulnerability checks for a particular type of mundane disturbance (violence, helplessness, personal) before failing a Vulnerability check for any reason. As with progressive Vulnerability Challenges, this will require adding checkboxes to the Investigator’s sheet. Once a Vulnerability check is failed, all boxes are erased and the process restarts. This can pair well with progressive Vulnerability Challenges to reflect the difficulty of developing resilience. You may choose to combine helplessness and self for simplicity. 


Violence: Injured, discover corpse, physically tortured

Helplessness: Friend or fellow Investigator injured/killed, long-term injury/debilitation, trapped or psychologically tortured

Personal: Business failure, argument with colleague/partner/spouse, ruined friendship

Mental Resilience, a Numerical Stat:

If your players do not mind tracking an additional value, your group could implement Mental Resilience as an alternative to Vulnerability, which would function like the Sanity stat. Each Investigator’s Mental Resilience value would start equal to their POW. Any Mundane Horror events or encounters would result in a Mental Resilience check with corresponding losses based on the published values in the rule and campaign books. You could elect to use the published Sanity rules to determine the effects of Mental Resilience loss, including temporary insanity and bouts of madness, or you could employ rules for Vulnerability following the loss of 5 or more Mental Resilience points. We have not used this in any of our games, as we prefer the more streamlined approach and think Trail of Cthulhu’s rule-set provides a much more effective two-stat method. 

Cognitive Turmoil, an alternative to Indefinite Insanity

We have not instituted any significant rule changes to Indefinite Insanity, aside from renaming it. We consider this a lasting effect of a Mythos Corruption ravaging an Investigator’s consciousness and resculpting their reality. A particularly evocative example of this can be found in HBO’s Lovecraft Country (spoilers) in young Dee’s experience.

Roiling internal turmoil. Credit – A. Currell

While we have not altered the rule, we do allow an additional treatment option for Cognitive Turmoil. In a campaign, Indefinite Insanity can pose a significant disruption to the game almost akin to a kidnapping where the player can lose agency for a substantial period of time. The idea that Cognitive Turmoil can be alleviated by a trip to an asylum or psychiatrist does not seem to fit this framework. Instead, you may call for Cthulhu Mythos rolls to either identify a source of treatment, such as a useful ally (psychic medium, conjure man, yatiri, or shaman) or provide direct relief via their Mythos knowledge. The presence of Cognitive Turmoil could set up an interesting side quest for aid during a campaign to save their friend. Instead of the affected Investigator being completely broken, the Keeper could impose intermittent or progressively disruptive episodes and/or penalties to reflect the effects of Corruption.  

Utter Corruption

Again, we have not changed the rules for Permanent Insanity, which still represents the irreversible endpoint of Mythos Corruption. At this point, the Investigator ceases to function as a player character. Whether they end up as an NPC threat or a pile of melted neurons can be up to the Keeper, but we have considered some alternative mechanics to determine the Investigator’s path. 

Most simply, you may allow the Investigator to make a POW roll once they reach Utter Corruption. If they pass, they can fight off the Corruption’s temptation, but still, experience a complete obliteration of their consciousness as they struggle against the unstoppable secrets of the universe. If they fail, they surrender to the Corruption, becoming an agent for the Mythos.

The optional 7th edition Mythos Hardened rule may also be employed to serve as a crossroads for your Investigators to determine the inevitable effect of Utter Corruption. Once they become Mythos Hardened, the Investigators can be asked to make a POW roll. If they fail, they have succumbed to Corruption and slowly begin down the pathway that cultists follow. If they pass, then they continue to fight Corruption until the bitter end. 

This option can be discussed with players before implementation as a means to potentially enliven their roleplaying experience but should include thoughtful consideration to avoid markedly disrupting group dynamics and upsetting players. Consider it an opportunity in Call of Cthulhu to gradually veer from “Neutral” and “Good” alignments towards “Evil”, as Investigators nihilistically abandon social and moral conventions.   

Exploring Corruption and Sanity

The Call of Cthulhu Sanity mechanic has influenced game design for nearly forty years, and many game systems provide varied takes on the original concept. The following games offer interesting approaches: Trail of Cthulhu, Fate of Cthulhu, Nemesis, Delta Green, Unknown Armies, and Cthulhu Hack.

We also found the episodes “Insanity in Call of Cthulhu” “Insanity in Lovecraft” “Insanity Mechanics in Call of Cthulhu” and “Mythos as Corruption” by the Good Friends of Jackson particularly thought-provoking and helpful in developing our concept of Mythos Corruption. 

A must-listen for fans of Call of Cthulhu, horror films & horror gaming in general.

We intend to continue exploring the topics of Corruption and Sanity in future posts and plan to bring you alternative Cosmic Horror and Corruption effects tables, as well as discuss the concept of Delusions.