Roleplayer #22, November 1990

Cthulhu Lives!

Lovecraftian Horror in GURPS

by David Ellis Dickerson

All those vile Ayshuns of Khopee rites aside, there are many people who like the GURPS Horror system, and would like to do some Lovecraftian adventuring with their characters. The only problem is that GURPS cannot produce any "official" Lovecraft material because Chaosium owns the rights. Chaosium has put out a lot of wonderful Cthulhoid adventures, but there will never be similar adventures written just for GURPS Horror.

But why should this stop the determined gamer? The Rules Police don't visit private parties, and there's nothing to stop you from buying a CoC module and running it with GURPS characters – nothing, that is, except the differences between the systems. How can you lose Sanity if you don't have it (as a characteristic) in the first place? How can you cast spells if you don't have any Magic Points? And how does a CoC skill like Credit Rating transfer to GURPS?

Don't worry. The two systems are remarkably similar when it comes right down to it, and with just a little word-juggling, you'll be ready to hunt down the Fungi from Yuggoth, whether you have the Call of Cthulhu rules or not.


There are a lot of characteristics in Call of Cthulhu, and some of them – such as Power – have many uses, but don't translate between systems very seamlessly. Here are the basics:

If you're translating investigators, subtract 2 from INT to determine GURPS IQ. Investigators tend to start with extremely high INT, which costs more than an average 145-point character can afford.

Use the CoC Appearance characteristic (APP) to find the character's Appearance in GURPS:

CoC Appearance GURPS Appearance
3-4Hideous (-4)
5-6Ugly (-2)
7-8Unattractive (-1)
13-14Attractive (+1)
15-16Handsome/Beautiful (+2/+4)
17-18Very Handsome/Beautiful (+2/+6)

A character's CoC Power is equal to GURPS Will, so compare Power with Intelligence, and modify it up or down with Strong Will or Weak Will, in order to achieve the correct score. (Example: A CoC character has INT of 13 and POW of 10. In GURPS, he has IQ 13 and -3 Weak Will.)

Power also determines GURPS Magical Aptitude. Power of 12-14 gives Magery 1; Power of 15-17 gives Magery 2, and Power 18-20 gives Magery 3.

In order to reflect the CoC rules, you may wish to give characters with Power of 15 or better the Luck advantage (15 points). 30 points of Luck does not exist in any Lovecraft story!

Movement should be figured from GURPS characteristics. Where the mode of movement is different (such as flying), translate the number directly: CoC Move = GURPS Move.

These rules are all right for the average investigator, but the monsters – particularly the tougher ones – may have characteristics which go much higher than normal. Here's how to deal with it.

High STR = High ST. Use the ST chart from GURPS Supers to determine Thrust and Swing damage. (For your convenience, this is reproduced at the end of this article.)

High CON = Give the creature HT 20, and as many hits as are listed under Hit Points after a slash, just as is done for animals. (Example: Zhar the Twin Obscenity has 100 CON and 100 Hit Points. In GURPS, he has HT 20/100.)

High SIZ = Any monster with SIZ of 35 or more has advantages for being larger. Divide SIZ by 20, and round off. This is how many full 100% increases in Growth (from GURPS Supers) the being has. For those without Supers, every time height doubles, ST doubles, Hits double, and Weight goes up eight times, while Move when walking or swimming (only) increases by 50%. However, you can assume that the creature stats as they are given in CoC (such as Hits, STR, and Move) already reflect the growth of the creature they describe.

Some beings (e.g., the Insects from Shaggai and the Fire Vampires) have SIZ of 1. Treat them as swarms (B143) with the given hits and doing the described damage.

High INT = INT higher than 20 is either IQ 20 or IQ (INT/2), whichever is higher.

High POW = For every full 10 levels of Power above 20, give the creature another level of Magic Aptitude. (So 30-39 gets +4, 40-49 gets +5, etc.) Aside from this, most creatures have enough Fatigue from their increased ST to fuel their Magic. In the rare case that a being's POW exceeds its ST, use the POW rating as if it were extra Fatigue, bought for spell-casting purposes. (This costs 5 character points per point of Fatigue; see GURPS Supers, p. 20).

Damage = Use the creature's ST. Tentacles and appendages do swing/crushing damage. Bites do thrust/cutt-ing damage (for regular teeth) or thrust/impaling damage (for fangs). Claws and pincers do swing/cutting, and spines and horns do thrust/impaling. Use weapons as they are described in the Weapons Chart. All other attacks (including blood drain, teleport, engulf, and so on) are considered Special; just take damage straight from CoC. CoC armor, by the way, is exactly the same as GURPS DR. Cthuloids have the same Passive Defense as a GURPS creature with equivalent hide.


Many skills have exactly the same name in the two systems. Unfortunately, CoC operates on a percentile rather than a three-die system.

While it's tempting to take the skill percentages and translate them into die rolls (based on the chart on B45), don't do it. Most investigators have terribly low success chances with their skills, and your translated investigators (who are professionals, remember) will have most of their skills at rolls of about 8 to 10 (25% to 50%). Instead, consider any skill of 50% to have been bought at IQ or DX level. For every full ten points above or below 50% (which means 45% and 55% are considered 50%), reduce or increase the IQ or DX roll by one.

Example: Festus Curtis, private eye, is a Call of Cthulhu character with INT 14, DEX 11, Handgun Attack 50%, Psychology 35%, Driving 20% and Latin 75%. In GURPS Horror, he'll have IQ 14, DX 11, and Guns (Pistol)-13 (bought at DX level, with a +2 IQ bonus), Psychology-13 (IQ-1, since 35 is only one full multiple often below 50), Driving (Automobile)-8 (at DX-3 level) and Latin-16 (IQ+1, since 75 is two full multiples often above 50).

As mentioned before, most skills have the same name (or at least very similar ones) in both systems. Here are the exceptions:

Credit Rating, in CoC, is supposed to represent impressive credentials and the ability to bluff through red tape. In GURPS, this might be Social Status or Military Rank, or a good personal Reputation, depending on the character background. Allow an appropriate status-type advantage worth +1 on reaction for every full 10% above the original Credit Rating score (15%). In addition, any character with Status of 3 or higher automatically has Savoir-Faire at IQ level.

Cthulhu Mythos is a Mental/Very Hard skill. It is used to identify Mythos creatures' involvement in a given scenario, and to identify particular creatures by name. Use the following modifiers:

In addition, there are certain skills which are unavailable in CoC which should be given to investigator characters. Journalists should get Journalism at IQ level. Dilettantes should have Savoir-Faire. And the GM should consider giving private investigators Shadowing, and police officers Criminology or Forensics (at TL6, of course).


CoC players lose Sanity under about the same conditions that GURPS players make Fright Checks. The more potential Sanity loss, the harder the Fright Check. To figure the exact penalty, total up the maximum Sanity loss possible. (GURPS characters don't get Will rolls to avoid Sanity loss, so if there's a listing such as 1d6/1 point, it means 6 possible Sanity.) Divide this number by two, and round down. This will give you the Fright Check penalty.

Example: An adventure states that reading a given book will invite 2D8 Sanity loss. The maximum Sanity loss from a roll of 2D8 is 16. Divide this in half, and you can tell any GURPS Horror players who read the book that they must make a Fright Check at -8. (Exception: seeing one of the Outer Gods usually invites 1D100 Sanity loss! Treat this as a Fright Check -20.)


You may use spells from either Call of Cthulhu or GURPS. Magic Point cost is the same as Fatigue cost, and damage translates exactly: 1D10 hit points' damage is 1D10 HT damage. You may change all "1D10" references to "2d-1," and all "1D8"s to "1d+1"s, but the point is that damage is lost at the same rate in both systems. Any spell which costs more than 12 Magic Points/Fatigue is as a Very Hard skill.

Monstrous Strength

Here's the strength chart (from Supers p.79) for strengths higher than 20:

212d4d-1 455d7d+1
222d4d 505d+28d-1
232d+14d+1 556d8d+1
242d+14d+2 607d-19d
252d+25d-1 657d+19d+2
262d+25d 708d10d
273d-15d+1 758d+210d+2
283d-15d+1 809d11d
293d5d+2 859d+211d+2
303d5d+2 9010d12d
313d+16d-1 9510d+212d+2
323d+16d-1 10011d13d
333d+26d 11012d14d
343d+26d 12013d15d
354d-16d+1 and so on:
+1d for each full
10 points of added ST.

And that should about do it. Using these rules, you can translate characteristics, skills, monsters, magic spells, and even insanity. In fact, about the only thing you can't do is beat the bad guys. But that's Lovecraft for you. Isn't it nice to have him in your campaign?

Our thanks to Chaosium for granting their permission to publish this article. Call of Cthulhu and its supplements are copyrighted by Chaosium, Inc., and Call of Cthulhu is a registered trademark of Chaosium.

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