Faerzress

Magic works differently in the Underdark. It settles in things. It pools with age in a sort of temporal gravity. It rushes through rivers and concentrates in pools. It calcifies deep in the stones. It awakens as lava. It crystallizes. The gems and metals of the earth are one manifestation of this process, but so are the densest rock and the vastest void, and so, on a much shorter timeframe, are some people. The duergar of Gretretka, notably, become harder over time. Magic settles in their bones.

"Faerzress" is the duergar word for magic, though surface dwellers knowledgeable of the different ways that magic behaves use the term to refer only to the magic of the Underdark. Duergar familiar with this use of the term disregard it as surface dweller idiocy. All magic derives from depth and density. Magic diffused is magic made impotent. The magic of the stones is deep. It ramifies with the slowness and sureness of rock pushing on rock pushing on rock since the beginnings of time. The movement of tectonic plates is a relative jig. "Faerzress" doesn't mean Underdark magic or duergar magic - it means allmagic.