The Shattered Gates of Slaughtergarde is an adventure module written by David Noonan and Mike Mearls which is setting neutral, with conversion notes for the Eberron campaign setting. It was released in 2006.
Centuries ago, forces of good and evil collided in a titanic battle at the mountain fortress of Slaughtergarde. Armies of mortals and angels and defeated a rampaging demon horde, banished its demon prince, and tore Slaughtergarde apart, hurling it back into the Abyss. However the destruction of Slaughtergarde was incomplete.
Parts of the fortress were embedded underneath the Valley of Obelisks, buried for all time. Now, malign forces have found Slaughtergarde’s dark chambers, and they toil ceaselessly to restore the magic gates that will reconnect Slaughtergarde to the Abyss. If they aren’t stopped, a new demon horde may emerge from the shattered fortress to rampage across the world.