The Lightless
From UnderlightWiki
The Lightless:
An apparently inhospitable realm. The drain of power from it has apparently resulted in being a pitch black realm, requiring other senses than sight to allow beings to function within it. It's original state is unknown, but presumed to be similar to our own Underlight... though likely notably smaller. It exists mainly as a series of tunnel like structures now, including the residences of its occupants.