Dark Knowledge of Cthulhu City

Exploring a graveyard under Arkham, detective Martin Harvesson glimpses a hideous goat-headed, cloven-hoofed monster gnawing on a human skull. Stricken with horror, Martin turns and flees blindly down the tunnel – only to emerge into the grand concourse of a huge subway station, a vaulted ceiling of marble arching high above him, and engraved on that mighty dome is the image of that same goat-faced horror…

In this interpretation of Cthulhu City, the nightmarish city of Great Arkham overlaps with our reality, but can only be entered and perceived by those who have been driven insane by contact with the Mythos. When an investigator loses Sanity, they enter Great Arkham for one day per point of Sanity lost. The investigator appears in Great Arkham in roughly the same place as they stand in the real world; so, if an investigator loses Sanity in Innsmouth, they arrive down in the docklands of Innsmouth port.

From the perspective of those in our world, the afflicted investigator might be wandering down the street, or curled up in a catatonic ball, or lying on the ground whimpering. However, Cthulhu City is objectively real on some level – characters who lose Sanity at the same time enter the same city, and can perceive each other and communicate, and retain full memory of their experiences when they recover.

Future Sanity losses keep the investigator trapped in Great Arkham for a few days; the second Sanity loss is two days per Sanity point lost, the second Sanity loss is four days per point lost and so on.

After this first exposure, they can spend a point of Stability to cross over from one side to the other, but they must wait at least one day before going back. Investigators who trespass across the boundaries of sanity and dream in this manner are hunted by the Transport Police; those who are carried across in the bosom of madness are not pursued in this way.

The Divided Party

While having the insane investigators wake up in Cthulhu City gives the players something to do while their characters are incapacitated, it does mean running two games at the same time. The trick here is to have the Great Arkham side echo the ‘real’ investigation, describing it as a surreal, nightmarish version of whatever the sane investigators are doing. Those tainted by the Mythos – the cultists, the sorcerers, the monsters wearing human faces – are figures of influence and stature on the far side.

Forbidden Knowledge

If visiting Cthulhu City gives insight into secret powers and hidden cults, and the investigators enter the city through losing Sanity or willingly spending Stability, then this brings the theme of forbidden knowledge to the forefront of the game. Maybe all tomes and unholy ceremonies, all unthinkable thoughts and blasphemous knowledge, all roads leading to the same dark city…


Trail of Cthulhu is an award-winning 1930s horror roleplaying game by Kenneth Hite, produced under license from Chaosium. Whether you’re playing in two-fisted Pulp mode or sanity-shredding Purist mode, its GUMSHOE system enables taut, thrilling investigative adventures where the challenge is in interpreting clues, not finding them. Purchase Trail of Cthulhu, and its many supplements and adventures, in print and PDF at the Pelgrane Shop.

1 thoughts on “Dark Knowledge of Cthulhu City

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