Greyhawk: Classics, Book 06 - Keep on the Borderlands

The keep has fallen on hard times.
Raiders batter the outer walls.
Hordes of goblins and orcs roam the countryside, attacking trade caravans and killing merchants.
A mysterious stream of dark-robed priests travels through the keep’s walls, heading out into the wilderness, never to be heard from again.
Amid all this trouble, a band of hired adventurers journeys to the Caves of Chaos to stem the tide of turmoil once and for all.
The borderlands may never be safe again.