In our current Masks of Nyarlathotep campaign, we have introduced Mythos Corruption as an alternative framework to the classic Call of Cthulhu Sanity mechanic. This modification to the existing 7th edition rules provides a slightly different means for tracking the effects of Mythos and mundane horror. By shifting the emphasis from deteriorating sanity to progressive cognitive corruption by incomprehensible forces, we have altered the effects of mundane horrors on our Investigators. Instead of the banal disturbances and terrors of our mortal world working in tandem with the Mythos to chip away at an Investigator’s mental health, these events lead to vulnerability and weaken a character’s resistance to the effects of Mythos exposure. In developing this simple mechanic, we use an existing characteristic, POW, and drew some inspiration from other investigative horror role-playing games, including Trail of Cthulhu.
POW – The Basis for Mental Resilience
In Call of Cthulhu, the POW, or Power, characteristic represents an Investigator’s force of will, and, per the 7th Edition, the higher the POW, the greater the character’s aptitude for and resistance to magic. In our games, we also like to use POW as a marker of mental fortitude, which may be used in rolls opposing torture or interrogation. This is similar to the Agent Statistic POW in Delta Green, which is described as “force of personality, motivation and psychic resilience” and represents the Agent’s ability to keep their head in a crisis and stand up to pressure.
The POW characteristic can help us conceptualize how certain individuals become cult leaders and sorcerers, while others function as servants or pawns. We imagine powerful enemies, like Edward Gavigan and Carl Stanford, possessing a very high baseline personal motivation or will, which might be driven by sociopathy, narcissism, or megalomania. Pop cultural examples could include Lex Luthor, Hannibal Lecter, or Voldemort. Such individuals have the strength of will to remain organized and lucid despite ongoing or complete Mythos corruption. They may even harness their psychic strength to drain will from others, which in turn fuels their increasing POW allowing them access to greater magical abilities.
Alternatively, others with elevated POW, including Investigators, may experience Mythos corruption, but possess the psychic and emotional fortitude to decline the lure of dark power. Instead, they experience the inevitable obliteration of their mind through Utter Corruption. Conceptually, this replaces the state of Permanent Insanity.
Individuals with lower POW may respond more submissively and erratically to Mythos contamination, and such examples may include Renfield, Peter Pettigrew, and Smeagol. In Masks of Nyarlathotep, two key NPCs (Spoilers) provide clear examples of this type. These individuals may lack the will to choose between submission or resistance to Eldritch forces.
Vulnerability – Disrupting the Investigator’s Psychic Defense
Vulnerability represents a break in the Investigator’s emotional fortitude with increased susceptibility to the effects of Mythos Corruption. We chose to use Vulnerability as a binary trait for simplicity rather than another numerical value to track during gameplay. An Investigator is either Vulnerable or not.
During play, the Investigators may experience or witness disturbing events, like discovering corpses, seeing colleagues die, or inflicting violence on others. Using conventional Sanity mechanics, these experiences of violence or helplessness could result in Sanity loss.
When the Investigators encounter a mundane horror in our campaign, they perform a POW check at Regular difficulty. If they pass, they experience no effect. If they fail, they become Vulnerable and will be at greater risk for Cosmic Horror (Bout of Madness) secondary to Mythos Corruption (Sanity Loss).
We track this very simply as an added checkbox on the character sheet. As with episodes of Cosmic Horror, we encourage our players to describe and role-play the effects of their failed rolls. This may manifest as rage, grief, loathing, or terror depending on the circumstances and the Investigator. At Keeper discretion, Vulnerability checks may occur concurrently with Mythos Resistance checks. For example, when a Ghoul mauls an Investigator, they may first perform a Vulnerability check for the act of violence followed by a Resistance check when the Investigator realizes the nature of the attacker. You may find the successive checks increase player tension in these moments.
A good night’s rest can go a long way, so we reset Vulnerability at the beginning of each new session or in-game day. Other Keepers may elect to delay the recovery depending on circumstances and environmental stresses.
Consequences of Vulnerability
Once a Vulnerability check has failed at any time during the session (or in-game day), the Investigator can experience Cosmic Horror with a single loss of 5 or more Mythos Resistance (Sanity) points the Sanity Rules. If an Investigator has not yet failed a check, they must lose 10 or more Mythos Resistance points in a single instance before experiencing Cosmic Horror. An emotionally rattled character may be overwhelmed by a Deep One encounter, but nearly all comers should be overwhelmed when seeing a shoggoth or Cthulhu. Due to the traumatic nature of Cosmic Horror, the Investigator will automatically become Vulnerable during the episode.
As mentioned above, we also like to consider POW as the Investigator characteristic used to resist the temptation of spreading Mythos Corruption. Some spells require POW expenditure, which we consider the surrendering of one’s own will (or psychic strength) to contact a deity, enchant an item, or rip apart time and space. By tying POW to increased risk of Cosmic Horror, the Investigator faces heightened stakes each time they consider casting a spell resulting in permanent POW loss. In our campaign, Irina has multiple factors to consider each time she wishes to open a Gate, which helps limit the use of this powerful spell.
Modifying Vulnerability Checks
In particularly disturbing circumstances, such as for the death of an innocent or another Investigator, the Keeper may choose to modify the difficulty level of the roll. Other circumstances which could call for modified difficulty include sustained torture or forceful interrogation.
If the Investigators have already experienced successive trying events during a session, you could modify the difficulty or add a penalty die to reflect the mounting emotional effects. Bonus dice can also be awarded to Investigators based on their occupations and backstories. You would not expect a surgeon and librarian to respond similarly to the discovery of a mutilated body. When using bonus and penalty dice, we remain open to player feedback, ideas, and suggestions.
Summary of Mundane Horror & Mythos Corruption Mechanics
Example of Vulnerability
During the exploration in the Peruvian highlands, Dr. Dibden and Irina discover a massive pit containing numerous decomposing corpses, which requires a Vulnerability roll. Irina rolls a 44 (POW 65) and steadies herself. Dr. Dibden, a veteran of the Great War and physician, receives a bonus die, but still fails his roll with a 67 (POW 60). Due to his Vulnerability, he suffers an attack of Cosmic Horror in the chapter’s climactic encounter, and survives thanks to the intervention of Irina, who still loses Mythos Resistance (Sanity), but resists the Cosmic Horror.
Recovering from Vulnerability
You may allow Investigators to attempt to recover from Vulnerability during a game session through different methods, including successful Psychoanalysis or Psychology rolls, which may only be attempted once each. We like to limit these attempts to once per session, and the effect of fumbles and pushed rolls could include an added Penalty die for all Mythos Resistance (Sanity) rolls during the remainder of the session. Alternatively, Vulnerability could persist into the next session. Extreme or Critical successes could remove Vulnerability. A resoundingly positive result may reassure the Investigator of their abilities and renew their confidence.
Our Experience with Vulnerability
Our players have thoroughly enjoyed the use of this mechanic at the game table. Normally, they bravely face down Mythos threats, but once they have failed their check, they begin to play more conservatively and cautiously, accurately representing an individual who has been rattled by a series of upsetting mundane events. The compartmentalization of Mythos and mundane disturbances has fostered a greater sense of authenticity during our campaign and consequently improved our role-playing experience. We do not expect Mythos Corruption and Vulnerability to replace the Sanity mechanics during all of our Call of Cthulhu games, but we believe it adds flavor to pulpier, lengthy campaigns, such as Masks of Nyarlathotep or The Two-Headed Serpent, and we look forward to continued use at our table.
We plan on bringing you an additional post discussing some optional Mythos Corruption and Vulnerability rules, which provide some suggestions for cumulative Resilience challenges, Indefinite Insanity (Cognitive Turmoil), and Utterly Corrupted Investigators, as well as other topics.