The basic ingredients of any Diplomacy variant is a scenario where several (usually at least five, preferably seven) more or less equal Powers take part in a conflict for domination of a geographical area. The geographical scale of this conflict is not of itself important, it could be a city (as in Mobtown), a country (as in War of the Roses), a continent (as in Abstraction), the planet (as in Mercator), the solar system (as in Apposition), and so on. Now this approach is fine for historical variants, the budding designer merely confines himself to those periods of conflict where a number of competing factions have been battling it out, and ignores human conflict which is essentially just two-sided (e.g. the Hundred Years War, English Civil War, American Civil War, Franco-Prussian War etc.). After all as the essence of Diplomacy is the ability to have a changing alliance structure, games where the number of protagonists is small are not going to be very exciting or practical.
There are several difficulties in adapting fantasy novels to this basic formula. First, although there may be a superficial appearance in the novel of a number of Powers battling it out (Elves, Dwarves, Orcs, Ents etc.) all too often the basic story often revolves round the age old Good vs. Evil storyline - a two-sided fight with restricted scope for diplomacy. Second, in fantasy novels the impact of individuals is often out of all proportion to their numbers - the effect that a Conan, an Elric or a Gandalf may have on a campaign may be far more important than the weight of numbers behind him or against him. Third fantasy novels are, in the end, novels. They are telling a story. While it is possible to place a Diplomacy variant in a fantasy world, if you want the variant to have the flavour of the plot, then you will need to make the rules very complex indeed.
Therefore the variant designer must make one basic choice before he gets very far in his new design - is he just going to use the geography of the fantasy world turning his game into essentially a map change variant (e.g. Young Kingdoms I (Elric) or Middle Earth II (Lord of the Rings)), is he going to tackle the problems posed by the characters and the plot of the novel which will usually involve Personality Units and special locations (e.g. Downfall (LotR) or Black Blade (Elric)) .or is the variant going to be some compromise between the two (e.g. Third Age (LotR) or Age of the Young Kingdoms (Elric)). When I designed my Young Kingdom variants, my initial attraction to the Elric stories was the idea of having a map which consisted of land masses clustered around a central sea, because I thought this would provide an ideal vehicle for Fred Davis's A/F rules. In Young Kingdoms II I endeavoured to improve the map balance, add a device for breaking stalemate lines and then threw Elric in for good measure.
One inevitable problem in trying to introduce characters from the book is that the ratio of Heroes to Powers is invariably not 1:1. Some Powers will have no Heroes or Personality Units, other will have several. Therefore you must either find counterbalancing Personality Units for the Powers without them (which involves stretching the original story), hive off some of the Heroes/Personalities to additional players (which can make the variant need so many players as to become difficult to get off the ground or introduce players with a lessened degree of involvement in the game), or just accept that the game cannot be equally balanced in terms of manpower.
A surprising number of the plots in fantasy novels also revolve around unequal conflicts - i.e. the forces of Evil are large and powerful, sweep all in front of them, but are ultimately defeated not by the armies of Good but by the Hero. This tends to mean that the Evil player(s) start off very strong (hence Mordor s 2A s in Downfall) which can make the game very imblanced, or the Personality Units end up dominating the game. The designer then has to come up with some restrictions on alliances in the game to stop the Orcs teaming up with the Elves and the Ents to crush the Gondor/Mordor alliance, because such an alliance structure would be contrary to the mythical historical premise on which the game is based. It should be noted that fanciful alliance structures are not necessarily thought to be a problem in regular Diplomacy.
When you consider all the chrome" which has been added to the various versions of Downfall of the Lord of the Rings (Personality Units, the Ring, Special Locations, Multiple Units, Cavalry Units, Special Rules etc.) you can perhaps understand why it has been through 20-odd revisions. Some versions seek to make the variant more like the book, some seek to make the variant more playable, and (I suspect) never the two shall meet.
Are these variants worth playing? Well, it depends. If you want to try out Diplomacy in a fantasy scenario then of course they re worth playing, but don t expect the game to be anthing like as balanced as a regular game. On the other hand, if you want to relive the tribulations of Frodo as he seeks to destroy the power of Sauron then either turn to computer games or better still re-read the book. All in all, fantasy novels do not make good subjects for balanced Diplomacy variants
Stephen Agar is obviously quite a fan of variant play and design.