GazBC1: Shimmering Lands: Last Beacon of Blackmoor - DM's Guide

From the Introduction:

Mystara BC 2300 is a time of immense upheaval. Just 700 years after the destruction of Blackmoor, the catastrophe is still fresh in the minds of those who survived. The Great Rain of Fire is still remembered (firsthand by some of the longer lived races such as the elves) and its effects are still felt across the world. Although the Great Rain of Fire’s destruction was beyond compare, its aftereffects were just as violent. Those that survived were subjected to the wasting disease. Life became painfully short as the flesh literally wasted away from the bones.