Exodus of the Underdark

Publisher's blurb:

In the isolated reaches of the Silver Marches, local communities have been beset by an increasing number of Underdark denizens. Drow and deep gnome raiding parties steal food and skirmish with local forces. Deep underground the reason for the sudden Underdark incursion becomes clear: the drow have lost the great city of Menzoberranzan. Beset by an outside force, they have retreated to caverns nearer the surface while they prepare to retake their city.

Who, or what has declared war against the drow? What hidden purpose do they hold? Brave the depths of the darkness to find the answers, discover clues of a greater evil behind the invasion, and help the drow to reclaim Menzoberranzan!