"From the creator's blog:
"Dark Dungeons is a free retro-clone role playing game- a game in the style of old-school 1990′s role playing games, back in the days when rogues were thieves, races were classes, and you could start your adventuring career exploring a few basic dungeons and end up travelling the planes as an immortal being of pure divine power.
For those of you who need further explanation, read on:
According to , a Retro Clone is a copy of a rule system that is no longer supported.
In international copyright law, it is not possible to copyright actual game mechanics, only the literary and artistic expression of the author(s) of the game when describing those rules. Therefore- providing one is careful to avoid infringing on the copyrighted expression of the original author(s) while doing so- it is possible to replicate the mechanical rules of an old game that is no longer available in order to provide compatibility with the old game’s expansions and adventures; and to replicate the playing experience of the game.
Dark Dungeons is a retro clone of just such a game- the game in question being a particular legacy version of the world’s most popular role-playing game.
However, despite the legality of replicating game mechanics, the name of that game is actively trademarked and direct claims of compatibility with that game could be considered to be infringing on that trademark beyond the bounds of "fair use." Therefore, to be on the safe side, this site never refers to that game by name. Instead it is referred to as "The Trademarked Rules."
Rather than writing a new game completely from scratch- which would involve inventing new terms for common concepts and so on, Dark Dungeons is an Open Game License (OGL) game that uses the System Reference Document (SRD) as a basis.
The System Reference Document is a set of role playing game mechanics published by Wizards Of The Coast. Dark Dungeons is derived from the rules in this document, and many of the terms and concepts are taken- by the terms of the Open Game License- from that document.
The complete System Reference Document can be found here.
The advantage of this approach is that although none of the text or mechanics of Dark Dungeons are copied directly from the SRD, Dark Dungeons takes the terms that the SRD uses for its mechanics and alters them so that they match the mechanics of The Trademarked Rules. That way when there are terms common to both the SRD and the Trademarked Rules, Dark Dungeons can take them legally from the SRD rather than having to invent new terms in order to avoid infringing on The Trademarked Rules usage of those terms""
Source: Wikipedia, "Retro_clone", available under the CC-BY-SA License.