As of 939AC, fourteen dungeons have been discovered, and more are uncovered with each passing generation. Only five of these have been conquered, despite countless attempts... most of which have resulted in failure death.
The dungeons are considered property of the different cities if they're within their land's boundaries. It's typically illegal to explore a dungeon unless the city officials grant permission, and cities have been known to squabble over ownership rights.
The entrance to a dungeon acts as a portal to a dimension separate from Eternia, where the Dungeon Masters rule. From great volcanic pits to massive deserts, nobody really knows what they're in for until they enter the dungeon's gate.
A Dungeon Guardian has absolute control over the Dungeon. They are essentially immortal, unable to be defeated and capable of altering reality within that pocket space. A Guardian's task is to prevent those unworthy of leaving the Dungeon with magical items.
They generally appear as twisted, speech-capable animals, and are rarely human in appearance.