Converting Achtung! Cthulhu to Trail of Cthulhu

Modiphius_logoby Christopher Smith Adair

I’ve been converting scenarios to Trail of Cthulhu since I started playing the game. I get to dig in to the scenario or rules, see how it works, and reinterpret it. There’s at least as much art as science to conversions. Although I constantly refer to the ToC rulebook’s helpful guidelines, I use them as advice rather than proscriptions. Recently, Modiphius Entertainment gave me another opportunity to play around, with their WWII setting, Achtung! Cthulhu.

When converting stat blocks, I consider the character’s function and place in the plot. I often lower or raise Health or fighting ability ratings from what a “by-the-book” conversion would and take care to assign adequate Magic or Stability ratings to sorcerers. For Three Kings, I wanted the pre-generated Investigators to be the mathematical equal of ones created normally. Starting with a baseline conversion, I discovered I needed to spend a number of extra points on each Investigator. This let me cover more abilities between them, as well as increase existing ones, establishing the sorts of roundly capable Investigators that typify ToC.

When converting the investigation itself, I look at the climax and analyze how it’s arrived at, working backwards to identify the crucial moments. Call of Cthulhu scenarios are often structured very differently from ToC ones, so genuine core clues might be few. The important thing is to account for anything necessary for the Investigators to find their way. I then look at all the clues and decide which abilities can be used to find them. If a clue isn’t a core one, I determine if there is a cost to find it. I try to provide numerous zero-spend clues and attempt to make sure that no ability is overly represented. Library Use and Interpersonal abilities can be especially tricky, so I often consolidate clues that originally required multiple rolls.

The Zero Point scenarios included random tables and determination of number of foes. I removed or modified most of these elements, allowing Keepers to set the pace and to stay truer to ToC’s design principles.


The scenarios’ writer, Sarah Newton, presented two design challenges: rules for vehicle combat and unit-scale skirmishes (including abilities to direct squads). Adam Gauntlett’s WWI scenarios for ToC provide some examples of vehicles at war, but I needed rules that were more expansive and self contained. My original draft hewed closely to the rules for disabling vehicles in the ToC rulebook, adding scaling Hit Thresholds and protective armor for the vehicles and personnel. Though these draft rules worked during playtesting, they weren’t entirely satisfying. Following revisions and a battery of playtests covering various situations, the final rules have a little more complexity, such as degrees of damage (which make vehicles harder to control) and crashes, while still feeling like GUMSHOE. The mass-combat rules from Heroes of the Sea were simpler to convert, mostly requiring figuring out how to model all the modifiers provided by troop quality and commander ability tests.

Modiphius Entertainment gives Mythos roleplayers a variety of options when it comes to choosing a system to play Achtung! Cthulhu. I’m honored to have been part of bringing the ToC version about. It gives players a thrilling setting for both Pulp and Purist games, with rules that can be used to run warfare in other times and places.

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