How the Mighty Are Fallen

"As the floating cities of Netheril hover peacefully in the air, the winds of intrigue boil all around them. The phaerimm continue to plot the downfall of the Netherese even as Karsus prepares for the casting of his greatest spell. Undead walk the land, driving orcs and humans before them.  And somewhere below the greatest nation of Faerûn, the Tarrasque awakens from a long slumber.  How the Mighty Are Fallen is an adventure designed for use with the Netheril: Empire of Magic boxed campaign setting (which is required to play this adventure)."

This source book was available as a free PDF from WotC's website.