From the back cover:
The Underdark Holds Many Secrets
A series of earthquakes and eruptions have rocked a normally placid land. Strange creatures sighted in the dark of night raise fears across the already disturbed countryside. As tales of a lost race that once warred with the drow begin to surface, only the boldest adventurers dare to descend into the shadowy darkness.
Deep Horizon is a stand-alone adventure for the DUNGEONS & DRAGONS(R) game. Designed to challenge 13th-level D&D(R) characters, it presents a long-forgotten civilization for heroes to discover and save, if they can.