It translates to a big improvement over SoU; in SoU, one of the “revolutionary” new tilesets was just a repeat of a previous tileset ... with SNOW on it. You have to admit, that was weak. But the generosity of HotU more than makes up for it. Since the Underdark is virtually just as vast as the Forgotten Realms itself, just underground, the new setting basically “doubles” the size of the gameworld available.

Conniving and traitorous, dark elves are unflinching in their devotion to the "Spider Queen," the goddess Lolth. Their society is steeped in mistrust, with each house hell-bent on ascension and power. You have unwittingly drawn the ire of the Valsharess, a cunning drow priestess with a formidable army at her disposal. The drow maiden's advisors have prophesied that you, just back from the Plane of Shadow, are the one who will destroy her.