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An Unofficial Conversion of Call of Cthulhu for GURPS in the Jazz Age

Image taken from https://geekandsundry.com/the-best-rpg-of-all-time-is-call-of-cthulhu/

The material presented here is my original creation, intended for use with the GURPS system from Steve Jackson Games. This material is not official and is not endorsed by Steve Jackson Games.


The Call of Cthulhu investigative horror role-playing game, based on the works of H. P. Lovecraft and his imitators, is produced by Chaosium who hold the licence. The Chaosium game is in its 7th edition at the time of writing, and uses the percentile skill system that will be familiar to Call of Cthulhu players and those who recall the Basic Roleplaying System (BRP). I heartily recommend the game and this conversion is certainly not meant as a criticism of the Chaosium original.  

Image from http://tvtropes.org/pmwiki/pmwiki.php/ TabletopGame/CallOfCthulhu

I use Chaosium's published campaign and scenario books for the actual GURPS Mythos campaign, using the conversion guidelines from David Ellis Dickerson's, Cthulhu Lives! Lovecraftian Horror in GURPS, published in Roleplayer No.22 (Nov. 1990). I have also reproduced a substantial part of the article in one of the downloads to the right.


 I was lucky enough to pick up a reprinted 1928 Sears-Roebuck catalogue and use this as my pricing and equipment list. I recommend this approach if you are able to acquire one. If not, there are numerous lists available from Chaosium sources if the GM wishes to check the contemporary prices for equipment. Using GURPS's Cost of Living, however, it is unnecessary to track minor purchases which seem appropriate for the character's wealth and status level.

Picture from http://site.pelgranepress.com/trail/files/ wallpaperinvestigator_TOC.jpg


Scenarios that I have designed and written for my campaign. Feel free to use them if you wish.

Digging Deep: Introductory Meshing Scenario

Digging Deep is a ‘meshing’ scenario, intended and designed to introduce a new group of players to the setting and system, and to enable the players to establish their characters’ relationships both with the other player characters and with some important non-player characters.

I recommend that players do not read this scenario unless and until the GM rules that learning all the spoilers for the NPCs is no longer a threat to the mystery of the campaign.

Digging Deep Handout

An Overview of the Setting


The classic setting for Lovecraft's Mythos tales was the first three decades of the twentieth century, with the 1920s forming the genre's core period. Most of his characters were from the Northeastern USA, especially Massachusetts.

Whilst many good different settings have been proposed and supported for games investigating the Mythos, this GURPS conversion is firmly rooted in the period 1919-1939.

The actual campaign will move characters all over the world, but the starting base is the Northeast USA, especially the cities of Boston and New York, with Lovecraft's Arkham and its satellite towns such as Kingsport and Dunwich a canonical home location.

Original Chaosium map of Arkham

My Campaign Philosophy


The style of my GURPS Mythos campaign is fairly gritty, as the use of the GURPS system might suggest. There is no reason why a more Pulp tone could not be taken, but I prefer a game where ordinary people are drawn into the horror that Lovecraft's Mythos represents.

I believe that the game works best when the players and GM are able to immerse themselves in the setting. Modern campaigns (and I include the 1920s in that)should not just be about playing linked scenarios, they should also include playing out social interactions and creating a believable world in which the characters live.

Image from https://br.pinterest.com/pin/183451384805956853/

The Downloads

Click on the titles below to open each pdf.


Character Creation Aids

Jazz Age Mythos Character Creation Guide

Jazz Age Mythos Genre-Specific Advantages

Jazz Age Mythos Genre-Specific Disadvantages

Jazz Age Mythos Genre-Specific Skills

GURPS Skill Cost Table
Click on graphic to enlarge

Just some useful tables for creating genre-appropriate characters. The unsuitable options have been removed, although each GM will need to make their own decision of what constitutes suitable.


Jazz Age Mythos Campaign Guide

A five-page pre-campaign guide to what I expect from my players, and what they can expect from me. 


Jazz Age Mythos Character Creation; Social Contacts Guide

A three-page guide to GURPS social advantages and disadvantages, setting out the options for building up social networks and companions. This is primarily intended for players new to GURPS.


1920s Setting

A campaign guide to introduce players to the setting of the USA in the 1919-1939 period. Includes guides to the economic and social background, including suggestions on how to handle the historical racism, sexism and homophobia that permeated the period.


Sanity and Insanity

At the heart of gaming the Mythos is the characters' sanity. I present an alternative way to handle the GURPS Fright Check, splitting checks into Stress and Sanity tests, and provide new tables. Also covered are the new Hidden Lore: Cthulhu Mythos skill and recovering sanity. 


Jazz Age Status and Jobs

A fairly lengthy treatment of Cost of Living, Ranks, Social Status and a 1920s-specific Job table. Included are 147 jobs appropriate for the era, broadly sorted in the same style as Chaosium's Investigators’ Companion Vol. 2.


GURPS Conversion of Chaosium CofC

A slightly edited copy of David Ellis Dickerson's 1990 Roleplayer article, adjusted in places to better suit my campaign.


Grimoire - to follow

I'll post this when it is complete. To be honest, most CofC spells can be handled by a GURPS GM without actually needing mechanical rules, just using the descriptions as presented in the Chaosium sources.


Click the typewriter to email, or email mike@saduria.co.uk
Feel free to email me with any comments, suggestions or even criticisms. I am open to any feedback and would especially be interested in hearing how you are using the campaign info.