Or, An Investigation into the Provenance of the “Open Cthulhu SRD”
One of the most controversial things to happen this year in the world of Lovecraftian tabletop RPGs has been the (non-)release of the Open Cthulhu SRD and the subsequent debate about its alleged Intellectual Property infringements.
I don’t think there’s any value in rehashing the lengthy (and generally insightful) debate that transpired on the Yog-Sothoth Forum (you must sign-up for free forum membership to see this content). But as a brief capsule summary anyway:
- A group of (AFAIK) unknown game designers created a d100 game of Lovecraftian horror which they aimed to release under the OGL; they posted links to their file on YSDC and other forums.
- Moon Design (Chaosium) promptly contacted both the creators AND the sites which hosted this link and bluntly asserted this material was in breech of their Intellectual Property and also that of various Cthulhu Mythos fiction authors. Moon also asserted that Open Cthulhu’s use of material from Mongoose Publishing’s OGL version of Runequest was also illegal since that latter work’s OGL was no longer valid.
- The Open Cthulhu group re-worked their SRD and claimed to have removed all the references to Mythos creations by any authors other than Lovecraft (whose solo works are now indisputably in the Public Domain) and replaced all content drawn from the Mongoose Renaissance OGL with material from the Legend RPG (which is 100% OGL).
- Lively debate followed about trademarks, the nature of the OGL, and other topics concerning Intellectual Property; Moon Design also asserted that they had re-assessed the revised Open Cthulhu SRD and found it to still be in breach of their IP (without citing specifics).
One of the things that I have observed about the online debate about Open Cthulhu is that most of the comments made about it’s use of the OGL are, to some extent, hypothetical (including my own).There are plenty of opinions which are qualified along the lines of “If Open Cthulhu’s claims are true about it being derived from valid OGL content then …” or “If Open Cthulhu has copy/pasted text from Chaosium publications then …” The reason these are hypothetical is because nobody has (to my knowledge) gone through the Open Cthulhu SRD with a fine-toothed comb to try to unpick its provenance. That is, to unravel the DNA of this controversial newborn? (stillborn?) beast.
So, I decided I would sit down and do just that — opening up the latest Open Cthulhu SRD side-by-side with all of the OGL documents it claims as its ancestors AND copies of multiple editions of Chaosium’s Call of Cthulhu rules (specifically 4th, 5th, and 6th editions).
The analysis took me a LOT longer than I thought — several days of concerted effort — but at the end of it I have gained a fair degree of understanding of what this beast is, and where it’s various parts seem to have come from. In this post I’ll summarize some of those findings.

Quick Summary (or TL;DR)
- Open Cthulhu’s claim that it contains no references to non-Lovecraft Mythos elements is basically justified
- The OGL ancestry of Open Cthulhu, as stated in its own OGL License seems legitimate
- Open Cthulhu draws in text from the Delta Green Agent’s Handbook (approx 16% of the Open Cthulhu wordcount), the Legend RPG (2% of the OC wordcount), Eldritch Tales and the Cthulhoid Bestiary for OSR (together 6% of the OC wordcount). Plus it has a smattering of text drawn from the Delta Green Quickstart
- While a significant part of the Open Cthulhu text comes from pre-existing OGL rules, two-thirds of its text is apparently new content.
- Some of the apparently new content represents rules/content that have no relationship at all with Call of Cthulhu (or any other Chaosium book I’m familiar with); however other pieces look like they are clearly intended to be “retro-clones” of pieces of older Call of Cthulhu editions.
- In terms of wordcount, about 31% of Open Cthulhu SRD appears to have a “Chaosium influence,” versus 35% of the SRD text which is new content but not inspired by Chaosium’s rules (as far as I can tell)
- Looking very closely at the Chaosium-influenced parts of the Open Cthulhu there is no evidence I could find of a direct “copy/paste” anywhere but plenty of examples where similar rules have been re-written using different words
Methodology
In order to analyze the provenance of different pieces of the Open Cthulhu SRD I decided to create a version of the OC text in a form that allowed me to “colour-code” different sections of text based on provenance. That is, whether it was directly (or almost-directly) copied from an OGL source, whether it was a wholly-original creation, or whether it was new wording attempting to reverse-engineer parallel text from Call of Cthulhu.
In the end my categorization scheme grew to recognize:
- Five OGL “flavours”: Delta Green Agent’s Handbook, Delta Green Quickstart, Adaptations of Delta Green rules, Legend RPG, and Miscellaneous OGL material (covering Eldritch Tales and the Cthulhoid Bestiary)
- Two Categories of original material: Entirely Original, Chaosium-influenced
- Two Miscellaneous categories: H.P. Lovecraft Quotations, OGL License Text
Categorizing (colour-coding) the different sections of the Open Cthulhu SRD was undertaken by comparing the relevant rules/resources text side-by-side with each of the relevant OGL sources and then against the different editions of the Call of Cthulhu rules. Where no source had any clear resemblance to any of those sources I assumed it to be original to Open Cthulhu.
Once each word of the Open Cthulhu SRD had been allocated into one of the categories, statistics were gathered of the total number of words that had been assigned to each category and the proportion they represent of the total Open Cthulhu wordcount.
For a full table of raw data in PDF form, click here.
The Structure of the Open Cthulhu SRD
The Open Cthulhu SRD is structured as shown in the diagram below, being split into two main sections — a Rules Section (aka the Player section) and a Keeper Section.

Overall the Open Cthulhu SRD consists of 61,014 words.
In terms of wordcount, the rules make up about 40% of the total file, with the Keeper section making up the bulk of the rest. There’s a small amount of intro and outro material at the front and back of the file respectively; the outro is made up of the OGL license text.

Overall Observations and Analyses
Looking at the Open Cthulhu SRD in its entirety, the breakdown of words assigned into each of the nine categories is as follows:
OGL Categories
Content from Delta Green Agent’s Handbook: |
9,520 words (15.6%) |
Content from Delta Green Quickstart: |
270 words (0.4%) |
Content extrapolated/adapted from Delta Green Agent’s Handbook: |
2,978 words (4.9%)
|
Content from Legend RPG: |
1,368 words (2.2%) |
Content from Miscellaneous OGL sources: |
3,837 words (6.3%) |
Original Content Catgories
Entirely Original Content: |
21299 words (34.9%) |
Chaosium-inspired Content: |
19052 words (31.2%) |
Miscellaneous Categories
H.P. Lovecraft Quotations: |
1736 words (2.8%) |
OGL License Text: |
954 words (1.6%) |
Use of OGL Material
It is clear that large sections of the Open Cthulhu rules section — especially the combat and sanity rules — are copied directly from OGL content in Delta Green. In some cases this material has been edited down to simplify rules or to omit particular parts altogether. For example, Open Cthulhu does not make use of the Character Bonds mechanic at all and all references to it have been edited out. Similarly, the combat rules in Open Cthulhu simplify some of the ways in which high-tech weaponry and/or militaristic weapons are represented.
On the other hand, there are places in Open Cthulhu where the core concepts introduced in Delta Green have been adapted or extrapolated. These are mostly minor, for example the renaming of DG’s “combat turn” to OC’s “combat round” or the expanded set of descriptions for how Lethal Damage affects different types of Mythos creatures.
Leaving aside Delta Green, it is clear that Open Cthulhu’s use of other OGL sources is more slight than might otherwise be assumed. In particular its direct inheritance from the Legend RPG is really very small and is mostly concerning game stats and character creation. The material which is sourced from Eldritch Tales is mostly short descriptions of different Great Old Ones, Mythos Creatures, and Eldritch Artifacts. Open Cthulhu’s borrowing from the Cthuloid Bestiary for OSR is similar, albeit on a much smaller scale — it draws upon descriptions of several Mythos monsters that are not mentioned in Eldritch Tales.
Original vs Chaosium-Inspired
As noted above despite its reliance on OGL sources for the bulk of its rules, more than two-thirds of the words in the Open Cthulhu SRD would seem to be new content (i.e., not derived from an OGL source). One of the more difficult aspects of this analysis was trying to separate out which parts of these non-OGL-derived sections represent entirely original ideas and which are intended to be similar-but-not-identical versions of rules from Chaosium’s Call of Cthulhu game.
It is very clear that both types of text appear in the Open Cthulhu SRD.
As evidence of entirely original content, I would point at things like Open Cthulhu’s rather intriguing new take on game stats for Great Old Ones (which it terms “Mythos Powers”). Rather than giving these entities full game stats, as Call of Cthulhu does, OC characterizes them more by the perils of mental contact and the raw Lethality of their physical forms (as a single game stat). Another idea that is (to the best of my knowledge) original to Open Cthulhu is the concept of Investigators needing to make a roll to become “immersed” in a Mythos Tome (i.e., truly believe it isn’t gobbledygook) before they can research its contents.
On the other hand, it is equally clear that there are many cases where Open Cthulhu has introduced rules which are obviously inspired by Call of Cthulhu. Examples which come to mind include the specific skills breakdown in Open Cthulhu which, while not a complete copy of the skills list in early Call of Cthulhu, preserves some of the quirkiness of those editions’ characterization of Investigator knowledge. Another clear example is the inclusion of a brief set of game rules for running Dreamlands adventures which seems closely analogous to rules in Chaosium’s H.P. Lovecrafts Dreamlands (and latter editions of the CoC rules).

A Breach of Copyright?
One thing that is very clear upon a close reading of the Open Cthulhu SRD is that it definitely *is* aiming to be a “retro clone” of pre-7th Edition Call of Cthulhu, in the sense that its rules are similar enough that they could be used to play supplements released for those editions. Arguably, given the slow and slight way in which rules change between different editions of Call of Cthulhu, this also means one could (without too much work) use the Open Cthulhu SRD rules to run more recently-published scenarios as well (which would make it a not-so-retro clone, as others have pointed out).
The big question to be answered when it comes to these “Chaosium inspired” sections is … whether or not they are breaches of the copyrights held by Moon Design? While I searched long and hard for a “smoking gun” which would show there had been literal copy/pasting of sections out of Call of Cthulhu (the most unambiguous form of breach), I didn’t really find anything that absolutely qualifies as such. Yes, there are stat blocks for monsters and for Lovecraft NPCs that use the same set of numbers against the same characteristics, but I’m not sure that qualifies as breach of copyright (since arguably the numbers and the names of the characteristics are both pieces of game rules … or at least that’s how they’ve been interpreted in the cases of other “retro-clones”).
In terms of narrative text in Open Cthulhu that is a literal copy of Chaosium text … I couldn’t find any examples. Of course there are *plenty* of examples where the Open Cthulhu SRD uses different words to describe effectively the same game mechanic found in Call of Cthulhu, but once again I don’t know that this is a copyright breach since rules themselves cannot be copyrighted only their textual expression. As an example of the use of different language to describe similar rules, here is the parallel text describing how Investigators gain percentiles in the Cthulhu Mythos skill when they suffer insanity:
Call of Cthulhu 6th Edition
(© Chaosium, quoted here under fair use) |
Open Cthulhu
(© Open Cthulhu, used under OGL)
|
Insanity and the Cthulhu Mythos
Insanity stemming from non-Mythos causes yields no Cthulhu Mythos knowledge. But each time an investigator reels from Mythos-induced trauma, he or she learns more of the Mythos, and this is reflected in the arcane Cthulhu Mythos skill.
The first instance of Mythos-related insanity always adds 5 points to Cthulhu Mythos. Further episodes of Mythos-induced insanity each add 1 point to the skill. |
Understanding the Cthulhu Mythos
Whenever an Investigator goes Temporarily Insane or develops a Disorder due to an encounter with the Cthulhu Mythos, he or she also gains some deeper understanding of the true nature of reality. On the first such occasion, the Investigator gains 5 percentiles in the Cthulhu Mythos skill; each subsequent occasion grants a further 1 percentile. This increase also serves to reduce the Investigator’s maximum SAN (and if current SAN is higher than this value, also causes it to drop to the new maximum). |
Provenance of Rules vs Provenance of Keeper-Resources
One thing that is quite clear when looking closely at the Open Cthulhu SRD is that there is a sharp difference between the way in which it derives its “rules” content versus its material for Keeper Resources (Mythos Entities, Creatures, Spells etc).
The Rules section borrows very heavily from existing OGL rulesets — most notably Delta Green. If taken in isolation, the rules section of Open Cthulhu has about 47% of its text derived from Delta Green with another 6% from Legend. That leaves 45% of the rules-section text which doesn’t come from any OGL source. This is mostly made up of skills descriptions, rules for skills improvement, sample weapons table, and examples of rules being used. While the example text used to illustrate and clarify rules is entirely original, most of the other areas identified above are a mix of Chaosium-influenced rules and entirely original rules.

Things are quite different when it comes to Keeper Resources, largely because the primary two OGL sources — Delta Green and Legend RPG — have no OGL content for Mythos entities. In the case of Legend it’s not a Mythos game, so that isn’t surprising. In the case of Delta Green, its own OGL makes it clear that those parts of the Agent’s Handbook are off-limits when it comes to OGL re-use. This seems to have forced the Open Cthulhu writers to (a) create more “new” text, and (b) seek out some alternative OSR rulesets which have small fragments of descriptive text about Mythos creations.
The provenance of the Keeper’s section of Open Cthulhu is broken down as shown below.
The small amount of “Delta Green Adapted” material shown here represents the text supplied for each Mythos creature which describes how it should be considered in terms of the Lethality combat mechanic borrowed from Delta Green. As can be seen, when it comes to Keeper resources, there seems to be a lot which is consciously inspired by Chaosium material, although it must also be said that there is a lot of really interesting material which seems quite distinct from how Call of Cthulhu handles situations (e.g., a different take on Great Old Ones, a more restrained approach to magic, and a set of nifty guidelines for making a brand new setting for Lovecraftian gaming and adapting the Open Cthulhu rules to work with it).
One thing for which Moon Design (and others) chastised the Open Cthulhu crew upon their first SRD release was the inclusion of Mythos elements which derive from works still under copyright (e.g., those of Brian Lumley, Ramsay Campbell, etc.). The Open Cthulhu team claimed to expunge all such mentions in their second SRD … so the question is, did they succeed? In general I would have to say that they did — there are no mentions by name of non-Lovecraft Great Old Ones, Mythos creatures, or Tomes. In fact the list of Mythos entities looks surprisingly similar to that published online for the German-language FHTAGN game (which probably isn’t surprising, since that game was held up during the YSDC discussions as an example of good practice in this area). The only qualifier I would place on my observations in this regard is that Open Cthulhu — like many other games — has rolled in several unnamed or generic Mythos creature categories (e.g., “Winged Servants” or “Hounds of Time”) which seem, to a greater or lesser extent, to be allusions to specific Mythos creations without actually claiming a direct relationship to those creations. It’s another “grey area” from an IP perspective I suppose.
Conclusions
The point of this exercise was to look at the Open Cthulhu SRD in detail to see whether its claims of OGL provenance check out, whether Chaosium’s claims of IP violation are backed up by examples, and whether Open Cthulhu was intended as a true “retro-clone” of older Call of Cthulhu editions.
I think that the analysis I’ve presented above answers many of those questions, although the question about IP violation is obviously a multi-faceted thing. I can certainly see why Moon Design has not been forthcoming with a laundry list of claimed copyright “violations” — since such a thing would be open to argument on both sides, which likely isn’t how they would wish to pursue the matter.
Above all this, however, I would have to say that a close examination of the Open Cthulhu SRD has surprised me in a couple of ways. Firstly, it’s really apparent that the writers of this system have put a lot of effort into it — I was expecting a shoddily slapped-together affair with little consideration for gameplay, but in fact the current rules show a deft and experienced hand at work judiciously picking pieces from several sources and assembling them carefully. The second way that the SRD has surprised me is in its aim to be more setting-independent and all-purpose than the thing it seems to be trying to copy — if the goal was to provide a basic engine to power a broad range of Lovecraftian d100 games, then it substantially succeeds, although obviously would need extrapolation to meaningfully work outside its stated range of eras (early twentieth century to modern day).
It’s a real shame that Open Cthulhu has wound up in such troubled straits, since otherwise it’s just the sort of thing I would recommend to game designers looking for an adaptable d100/Lovecraft base from which to build their own creations. As it is, though, it would be a brave person that took on such an endeavour . . .
[Raw data for the statistical analysis available here; marked up version of the Open Cthulhu SRD for peer-review available on request]
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