The objective of The Eidolon is to successfully navigate through all of the game’s levels, defeating the dragon guardian at the end of each level. The player navigates through each maze and collects energy orbs, which come in four different colors (red, yellow, green and blue). Along the way, various enemies wake up and attack the Eidolon, attempting to absorb its energy. Each enemy is a specific color and can only be destroyed by firing orbs of that color, or by using special abilities. Some enemies also fire orbs at the Eidolon, which can absorb them if set to the same color.
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
Green orbs, in addition to dealing damage to green enemies, also have the power to transform other enemies into different kinds of enemies, usually changing the enemy’s color as well. Blue orbs can freeze enemies temporarily, giving the player a momentary advantage in a fight.
Each level contains three diamonds, each guarded by an enemy of a specific color. After defeating each of these enemies and collecting the diamonds, the player can proceed to the dragon that guards the exit. There are eight levels in a game. The game is over when the player has successfully defeated the final dragon or runs out of lives.
The player discovers the Eidolon, a strange 19th-century vehicle in an abandoned laboratory. As the player investigates this device, he is accidentally transported to another dimension and is trapped in a vast, maze-like cave. The creatures in this cave, sensing the energy emanating from the Eidolon, are awakened from a long slumber, and the player soon finds that his only chance of survival lies in this mysterious vehicle and its powerful energy weapon.