From the back cover:
"Hellgate Keep - a name that once made the boldest of heroes shudder in fear - was long ago known as the fair citadel of Ascalhorn, a haven for elves and humans. Then a demonic horde of baatezu and tanar'ri destroyed all remnants of real civilization and turned it into a den of malevolence. Today, the citadel lies ruined. The Mistmaster and the Harpers destroyed the keep and ended its centuries-long reign of evil in the autumn of the Year of the Gauntlet (1369 DR)...or so they believed.
With the coming of the following spring, many scavengers streamed to the Upvale to uncover treasures and artifacts long hidden by the tanar'ri of Hellgate Keep. They went forth assuming they could find easy plunder. They were wrong.
Prepare to meet a new race of creatures spawned both of Faerûn and the Lower Planes. Uncover the corruption of an elven house that was ancient before Myth Drannor ever rose. Find out what survived the havoc visited upon the Keep, and who or what leads the survivors to a new lair. All this and more awaits amid the depths of ruined Hellgate Keep!
This stand-alone 32-page adventure is set within the northern wilderness of the Realms. It easily fits within an existing FORGOTTEN REALMS campaign, though it can also be incorporated into any other AD&D campaign world with minor modifications."