When I do play testing for The Cthulhu Hack, I often turn to other publishers for their adventures. As I noted in A Call from the Trail, Chaosium and Pelgrane do some good stuff – and it’s pretty easy to convert on the fly.
Someone asked whether any conversion rules existed for Call or Trail of Cthulhu. I see fuller consideration of conversion as a much bigger project, but for the time being I have a rough-and-ready approach.
Conversion should be rough, unless you’re looking to convert pre-gen characters or major NPCs. If the investigators interact with a non-player character for just a moment, you don’t need to spend time coming up with anything more than appearance and a tic or two.
Trail of Cthulhu
Because Trail of Cthulhu doesn’t have characteristics or attributes, you need to extrapolate strength and expertise from Abilities. VERY roughly, look to see whether the character has more points assigned to Academic or Interpersonal Abilities. If the former, give them more Flashlight dice; latter, give them Smokes.
For Hit Points, divide Health by 5 to see how many HIT DICE they have (rounding up).
You should look to General Abilities and work out where the character has more points spent. This is probably way too complicated
- STR (physical power and performance): Athletics, Scuffling, Weapons
- DEX (accuracy, coordination and bodily control): Driving, Firearms, Stealth, Riding
- CON (resilience and fortitude): Fleeing (it isn’t necessarily how fast, but how long you can run), Health (this is really rough, as a guide only – as characters will dump points here and in…), Stability
- INT (knowing technical stuff): Electrical Repair, First Aid, Mechanical Repair, Psychoanalysis
- WIS (instinct about the right or the wrong moment): Conceal, Filch, Preparedness, Sense Trouble
- CHA (exerting your presence or hiding it): Disguise, Hypnosis, Shadowing
For an NPC, see what Abilities the character has and note those with higher points assigned. If a character attempts to leverage the NPC for assistance, they would provide Advantage on those topics. If a character seeks to ‘beat’ an NPC with notable point allocations in any Ability, a Disadvantage would be in order.
Call of Cthulhu
Call of Cthulhu has a history in the Basic Role Playing system where you used to categorise skills into things like Communication and Academic. I did the same with 7th Edition – see the linked character sheet.
Again… VERY roughly, you can look to see how many points have been spent and convert to Flashlights and Smokes. Many skills I’ve categorised as Instinct fall into Smokes; a few of those and Learning are Flashlights. The rest – Training – cover physical stuff.
Saves you can extrapolate from characteristics by dividing them by 5.
I would suggest for a quick NPC, divide Hit Points by 6 to see how many HIT DICE they have (rounding up).
If a character significant enough to have skills, PCs will suffer Disadvantage against them if the skill is used as a threat. I’d suggest only do this if the NPC has a skill of 50% or higher.
For example, if the character has Persuade 60%, the investigator will make WIS Saves at Disadvantage if the NPC tries to deceive them.
For spells and unusual abilities, check the Spell list and existing Creatures for close equivalents.