The Eidolon was one of two games that were part of Lucasfilm Games' second wave in December 1985. The other was Koronis Rift. Both took advantage of the fractal technology developed for Rescue on Fractalus!, further enhancing it. In The Eidolon, Rescue's fractal mountains were turned upside down and became the inside of a cave.
In addition to common cassette formats, the Atari and Commodore C64 versions of the game were supplied on a flippy disk. One side had the Atari version, and the other had the Commodore 64 version. The Atari version required an XL/XE with 64kB or more memory.