Source: Wikipedia, Mystara , available under the CC-BY-SA License.
Mystara is a campaign setting for the Dungeons & Dragons fantasy role playing game. It originated as the Known World, a semi-generic setting used in early adventure modules, first mentioned in the Module X1, Isle of Dread, which was expanded upon in various D&D modules and sources, particularly a series of Gazeteers.
Mystara began as several semi-independent projects by different teams of writers who were each assigned to the task of developing different cultures and nation or nations that would exist in the fantasy world that was supported by Dungeons & Dragons at the time. Their work was gathered and compiled, blended, and combined to form a fantasy world, Mystara. Although it has officially been dropped from production by its creators, many fans continue to develop and evolve this fantasy setting jointly, continuing its original theme of group development.