From back of box:
"Many brave souls have fallen in quest (sic) of the mystic "Black Onyx"... Dare you risk your life for the mystery gem?"
The player can create a five-member party (4 members on GBC), or can ask friendly encounters to join the party to fill empty slots. The party will explore the dungeons under the town of Utsuro (ウツロ) to obtain the legendary Black Onyx. Doing so will break the curse which binds the town in eternal night.
The combat system has no concept of distance, so there are no bows or similar missile weapons. There are melee weapons, shields, and armor, however. The hospital in town can offer "examinations" which display the PCs' stats.
There is a bank in town where the heroes can deposit money. This protects money from thieves; but there is no interest on the deposits, since there is no time system.
The labyrinth under the town has several entrances, each hidden in one of the locations of the town. The labyrinth has six floors corresponding to the six colors the computer can display. These must be solved in the proper order (which depends on the platform) in order to create the stairway to the Black Tower, where the Onyx resides.
Source: Wikipedia, "The Black Onyx", available under the CC-BY-SA License.