Based on a playtest on The Enemy Within with: Guardkeeper: Lev Lafayette., Alexandra Donato by Julian Dellar., Fritz Snarble by Sam Sexton., Sven Halleck by Michael Cole., and Renate Hausier by Paul Smith
Background
Mouse Guard is a comic series written and illustrated by David Petersen. It portrays anthropomorphic mice who live in a parallel medieval era. The Mouse Guard is a group of especially brave mice who protect the borders, ensure the paths between colonies are kept open, relay messages and so forth. Mouse Guard the roleplaying game is co-authored by David Petersen and Luke Crane and is based on a simplified version of the Burning Wheel RPG, which Luke also wrote. Mouse Guard received the Origins award for Best Roleplaying Game for 2009. The comic was the winner of "Best Publication for Kids" Eisner Award, for Mouse Guard: Fall 1152 and Mouse Guard: Winter 1152. Personally, I don't really see how it's really for "kids".
The Enemy Within campaign is a series of adventures for Warhammer Fantasy Roleplay (WFRP) and was originally published by Games Workshop in the late 1980s and was voted the best RPG campaign of all time by Casus Belli magazine. The campaign as a who consists of six campaign packs; "Mistaken Identity", "Shadows Over Bögenhafen", "Death on the Reik", "Power Behind the Throne", "Something Rotten in Kislev" and concludes with "Empire in Flames". The last two supplements have not been as well received as the first four, and are not used in this cross-over story.
Conversions: Broad Principles
Using the Mouse Guard RPG with The Enemy Within Campaign obviously has numerous points of difference, but quite a few surprising points of similarity.
The obvious differences are setting based. Firstly, the use of anthropomorphic mice vis-a-vis 'standard' fantasy characters. This is not as much a difference as perhaps initially expected because the involvement of non-humans is quite modest in The Enemy Within. More importantly is the conversion of the mundane into the murinae. Humans become mice, horse-drawn carriages become rabbit-drawn (quite a bumpy ride), various animals of husbandry (sheep, cow etc) are right out of the equation, as are cats and dogs. Instead, as the Mouse Guard rulebook states, mice uses bettles, bees and crickets.
Secondly, is the prevalence of magic in The Enemy Within compared to Mouse Guard. Again, this is less of a problem that initially expected. Although the default Mouse Guard setting is pretty much non-magical in the sense of "flash-bang" nonsense, there is a sense of "magical realism" being involved. Converting magical effects of this kind In The Enemy Within is less difficult than imagined.
Social and technological issues require more work. Mouse Guard's setting is, as the dates indicated, around the 12th century. For all intents and purposes, the background of The Enemy Within is roughly analogous to the 16th century. This four hundred year difference is significant in terms of population, trade, equipment and so forth. On very particular example is the existence of weaponry such as the blunderbuss, which is - not surprisingly - completely absent in Mouseguard.
Blunderbuss: Missile Against short-ranged weapons, your Attack action counts a s versus action, rather than an independent, against Attack and is independent against a Maneuver. Long Range +1D to maneuver against normal, spear, or thrown weapons. Hard to Defend Add +4D (!) to your Attack versus Defend. These are relatively rare and require a Resourced 3 test to find and purchase.
Somemouse from Somewhere
In Mouse Guard, each mouse town or city has a cultural skill and trait. These can be converted to the Empire without much trouble.
Altdorf Skills; Administrator, Loremouse. Trait; Inquisitive
Averheim Skills; Apiarist, Harvester. Trait; Hardworker
Bechafen Skills; Hunter, Scout. Trait; Defender
Bogenhaffen Skills; Boatcrafter, Haggler. Trait; Generous.
Carroburg Skills; Carpenter, Laborer. Trait; Calm.
GrunburgSkills; Cartographer, Miller. Trait; Curious.
Hergig Skills; Fighter, Militarist. Trait; Cunning.
Middenheim Skills; Deceiver, Stonemason Trait; Independent.
Nuln Skills; Armorer, Smith. Trait; Fearless.
Pfeildorf Skills; Baker, Potter. Trait; Quiet.
Salzenmund Skills; Persuader, Weather-Watcher. Trait; Suspicious.
Talebheim Skills; Cook, Glazier. Trait; Stoic.
Ubersreik Skills; Pathfinder, Scout. Trait; Graceful.
Waldenhof Skills; Harvester, Survivalist. Trait; Quick-Witted.
Wolfenburg Skills; Archivist, Healer. Trait; Clever.
Wurtbad Skills; Insectrist, Carpenter, Trait; Determined.
Other characters can be extrapolated from the above. The general principle of "two skills, one trait" applies.
Magic and Religion
Magic and religion plays a big part in Warhammer's The Enemy Within. It is much less of an issue in Mouse Guard. With regards to the former, scenario conversion demands that the mundane take over from the supernatural, which isn't too difficult. One does not to have crawling chaos in order to put a sense of awe and fear into a mouse! For example, a key antagonist in Shadows over Bogenhaffen is the use of rather serious demon and their continuing interest in the town. This is simply converted to no less to an nested owl that lives on the outskirts of town. The narrative remains consistent with tomes on demonic summonings converted to the summoning and control of owls.
Likewise, the most common protagonists in the Warhammer version; everything from mutants, goblins, beastmen and all and sundry can be replaced with "plague mice", which should show all the symptoms of the equivalent of bubonic plague and other delightful infections. They gain an additional degree of horror due to the infectious nature, as any mouse with even Healer 1 should be aware of. There is, of course, no known cure so that mice that come in contact with "plague mice" had best be very careful indeed. The possibility of contracting the disease when in casual contact with a plague mouse or any place where a plague mouse has recently been for an extended period (e.g., a sleeping nest) is approximately 1 in 3.
In the Warhammer version of the scenarios, there are a number of powerful religions such as The Cult of Sigmar. In the Mouse Guard version of The Enemy Within the paranormal veneration of these deities are converted in more secular orders; just as numerous, with equally sufficient devotion on behalf of its members, although their adherence will be to an ethical rather than theological orientation. This applies equally to the various Chaos cults, although their attraction of supposed magical powers is somewhat greater.
Actual Play
Actual Play incorporated the design decisions above with a very high degree on congruence.