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.