This detailed campaign module proposes that within the sphere of the "Known World" is another world called the Hollow World, a sub-setting for the Mystara campaign world. Hollow World acts as a living "cultural museum" for the planet, forever preserving cultures and races that are near extinction on the surface world.

This box set includes the following:

• A 128-page softbound Dungeon Master's Sourcebook
• A 64-page softbound Player's Guide book
• A 32-page softbound Adventure Book
• Four single-sided poster maps

The books include information about new monsters, elves, dwarves, character classes, pirates, and immortals.