Helstar
Helstar is a magical city that is the setting of the adventure Assault on Helstar by Phil Schulz. The city and its surrounding area, including highways and railroads, a forest, and a peak known as Helstar Mountain, magically appear at rare intervals in different time periods and on multiple worlds including Earth and Eamon. Prior to its appearance on Eamon, the last recorded sighting of Helstar was a thousand years earlier near a small village in Africa. Schulz describes Helstar as an "enemy city", and the adventurer seeks to destroy it using a pair of magical gems.
The city and its surrounding area seem to originate in 20th century North America, based on the presence of contemporary highways and railway lines, a train station built in the 1940s, a crashed 1967 Mustang automobile, and various other items.
Helstar's behavior is similar to that of the fictional Scottish town in the musical Brigadoon which magically appears only once every one hundred years.