Mythos Corruption, an alternative to Insanity

The inexorable grasp of Mythos Corruption. Image by Andrey Grushnikov


The Sanity mechanic dates back to the original version of Call of Cthulhu from 1981 and has remained a fundamental aspect of the setting providing Keepers and players a means to convey and roleplay the effects of horror in the game. Many other game systems have incorporated, adapted, or been inspired by this concept. Throughout our MoN campaign, we have been challenged to reframe the “Sanity” mechanic to more convincingly reflect its role in both the campaign and the broader Mythos world. 

Part of the impetus arose from recurrent concerns about the depiction of mental illness and insanity portrayed by both the players and the NPCs.  Our playgroup consists of health care providers with numerous close, personal connections to individuals with mental health challenges. We sought simply to modify the basis for the Sanity mechanic without substantially altering the rules. In doing so, we intended to bring a greater level of comfort to the table and freedom to explore the effects of the Mythos without any additional baggage. We share this only as a suggestion, not as a mandate. In no way, do we seek to undermine the current system, which we have always and continue to thoroughly enjoy, but to provide an alternative framework, which may be employed at a group’s discretion.

Mythos Corruption and Resistance:

Corruption taking root at a cellular level. Image by Zeiss Microscopy.

In this framework, we begin by modifying some terms. Rather than degrading towards Insanity by loss of Sanity Points, a player (or NPC) will experience Mythos Corruption by depletion of Mythos Resistance. Instead of failed Sanity rolls leading to mental instability, we will describe the continued effects of contact with the Cthulhu Mythos as a form of progressive infection or psychic infiltration by an outside force. Ultimately, though, whatever “corruption” mechanism you and your players choose to imagine it only serves as our weak-minded mortal means of trying to understand the Mythos and its effects. 

The amount of Mythos Resistance lost reflects the degree of psychic damage inflicted by the Mythos exposure. This may be a gradual process with repeated, recurrent episodes leading to gradual corruption, which can help explain how Ghouls or Deep Ones may develop from ordinary humans. In the event of a horrific exposure to a terrifying monster or deity, you can understand how the overwhelming onslaught could breach a character’s Mythos Resistance and incite a profound response.  

Cosmic Horror: 

To reflect this, we have again altered the terms renaming Temporary Insanity as Cosmic Horror. At our table, we make slight modifications in the rules for our own game, which occur during Phase I, or “Bout of Madness” in the original rules, which we have termed Psychic Detonation. This terminology evokes a cascade of changes unleashed in the Investigator’s consciousness by exposure to the indescribable terror. 

Psychic Detonation:

A raging Psychic Detonation. Art by Maxwell Hamilton.

At our table, a Psychic Detonation lasts 1d10 rounds and the player retains control of their character. The effects of the Psychic Detonation can be determined in several ways, including a roll on a table, dealing cards off an effect deck, or Keeper/player choice. Our players greatly enjoy getting to roleplay the effects of their Psychic Detonation, as it allows them to behave wildly, indulge in dangerous behavior, and savor an intense moment. If we feel like they might be underplaying it, we will provide some suggestions or introduce a random element to push them further along. 

Unsettled Reality:

By actively participating in Phase I, the Investigator can transition smoothly into portraying Phase II, or Underlying Insanity in the original rules, which we have termed Unsettled Reality. In this phase, the Investigator remains at risk and follows the same rules as for Underlying Insanity. To mitigate the effect of Unsettled Reality during extended chapter play in campaigns like MoN, we elect to allow this state to last either 1d20 hours or until the Investigator has a safe and peaceful sleep. If you wish to apply these rules to shorter games or one-shots, you may choose to extend the Unsettled Reality for the full duration of your play.  

The effects persist with an Unsettled Reality. Image by vulcanjedilantern

During the Unsettled Reality phase, the Investigator will be at risk for recurrent Psychic Detonations as per the published rules. You can encourage the players to respond to any non-Mythos stimulus per their last Psychic Detonation. For example, during the study of the Liber Ivonis, Lucia experienced a Psychic Detonation, which resulted in a randomized episode of hemophobia. She consequently chucked Tip’s unopened bottle of Gould Campbell port through the hotel window. After recovering from her initial episode, exposure to any blood or other red liquids could either a) trigger another Psychic Detonation or b) offer an opportunity to roleplay an extreme response to the stimulus. 

Summary of Modifications:

In summary, we have introduced the following changes to the published rules in exchanging Mythos Corruption for Insanity:

Temporary Insanity (Published Rules)

  • Phase I – Bout of Madness (1d10 hours) – player surrenders control to Keeper
  • Phase II – Underlying Insanity (session/chapter duration) – control returns to player

Cosmic Horror (Corruption Alternative)

  • Phase I, revised – Psychic detonation (1d10 rounds) – player retains control
  • Phase II, revised – Unsettled reality (1d20 hours) – player retains control

Mythos Resistance Gains:

Victories and the successful pursuit of knowledge may offer temporary reprieve. Art by Glen Edelson

Cosmic horror reflects a complete rewiring of the Investigator’s brain by exposure to Mythos Corruption. Only by overcoming Mythos plots and threats can Investigators potentially restore their Resistance (Sanity) as they recognize that they may indeed have some shred of influence on these incomprehensible forces. Keepers may elect to allow players to recover Resistance by resting in safe and secure locations where they can be distracted from the terrors of the Mythos and restored by mundane pleasures. 

When thinking of asylums and private care, we reflect on Sarah Connor in Terminator 2; she was not insane, she was informed. Instead of willingly undergoing treatment, she focused on her resistance to future threats while institutionalized. You may imagine the same effect for your Investigators. This can also be reflected in the Resistance/Sanity bumps with skills increase to 90%. As your players become more able they feel empowered to resist Mythos corruption. From our perspective, the framework of Mythos Resistance provides a more plausible explanation for point recovery than Sanity. 

Framework Complications:

You have likely recognized that the Corruption framework is not a perfect substitute for the elegant Sanity Mechanic, which covers all horrors, from mundane to cosmic. By shifting Sanity to Mythos Resistance we have excluded the terrible effects of discovering corpses, witnessing a murder, and undergoing torture. We address this shortcoming with Vulnerability, an additional, simple mechanic discussed in our follow-up post below.

Our game table has found that this tweaked approach to Sanity has enriched our current campaign by focusing on the relentless effects of the ever-present Mythos, and makes the results of victories more plausible.  If your Session Zero reveals any concerns about the depiction and portrayal of insanity (and/or mental health issues), you may wish to consider presenting this framework to your table as an alternative option. 

Part 2 – Vulnerability & Mundane Horror

4 thoughts on “Mythos Corruption, an alternative to Insanity

  1. Ozyo says:

    As someone who works with a mental health and mental illness charity and has felt the friction between CoC’s portrayal of sanity and its complexities/sensitivities in the real world this is a system I would love to try some day.

    It’s early days for me as a Keeper; I’ve just got my head around chases and magic while preparing a second minature campaign! However, once I’ve got more confidence in everything, this is a system I would definitely try to mix things up and move things towards slightly more comfortable territory (not too comfortable mind!).

  2. Danny Stevens says:

    Hi folks,
    I love this change in nomenclature.
    I am ‘growing’ an RPG system and am preparing a freebie release of a sanity mechanic. I was wondering if, with siutable creditation, I could shift to using your nomenclature for that?
    As for vulneribility I have a mechanism called ‘The Shivers’ that is a value that is tracked. It comes and goes fairly quickly but makes the character increasingly vulnerable to shock. Things like psychic vampires manipulate this to help feed on their victims. Nothing than a strong coffe in a room full of friendly faces wont fix though.

Comments are closed.