One of the dangers in the Underdark is the Drow which inhabit the deep zones with their hidden cities ruled by their iron-fisted matrons. Outsiders traversing too close to their cities are equally as likely to be killed on sight as they are to wind up being a sacrifice to a priestess.
However, it is not the smokey charcoal-ed skinned visages of the Drow that fill me with fear. There is a small group whose complexions are of a pale bone white and whose eyes are the red hue of blood. These are the ones you need be worried about encountering for it would be better to turn your blade upon yourself than be captured and subjected to their company.
I only met half of the man that remained after encountering one of them and he lived only long enough to tell me to beware the palest Drow that wear an emblem of a red spider upon their chests.