From the Back Cover:
The Shadowdeep is a vast realm, so vast it contains entire worlds, great nations, lost cultures and incredible secrets. A few brave people, even entire communities, have faced the Shadowdeep and triumphed, some even reaching down to the Hollow World, the land of the red sun, and some instead reaching up from the Hollow World to the strange Outer World of day and night.
Threshold magazine presents in this issue the secrets and the treasures of the Shadowdeep, for you to discover and take. But not all of them, because the Shadowdeep is so vast you could roam it for your entire life, and never return to the worlds of the light.