Wikipedia on the Zork series:

Zork is set in a sprawling underground labyrinth which occupies a portion of the “Great Underground Empire”. The player is a nameless adventurer whose goal is to find the treasures hidden in the caves and return alive with them. The dungeons are stocked with many novel creatures and objects, among them grues and zorkmids. The Zork universe and timeline has been extended by several of Infocom’s other works of interactive fiction.