Faerûn's Underdark includes drow cavern-cities, sunken aboleth strongholds, entire realms of mind flayers, and wondrous vistas shrouded in everlasting darkness. The locales described here do not represent an exhaustive list of Underdark realms. Uncounted hundreds - perhaps even thousands - of deeply buried caverns and lightless cities exist beneath Faerûn, and virtually every surface-world dungeon seems to connect to murky depths from which all sorts of horrors can spew forth.
Absence of light results in darkness. So, what results from the absence of Good? The Underdark is a place with very little of either.Mordenkainen