Dark Abyss

The cave system that serves as the first half of the Underworld is often considered a 'dark abyss' because of the difficulty with which even the most accomplished explorers have in navigating it. The darkness is a tangible force that inhibits any light source that seeks to penetrate its depths, and a person definitely needs to see in order to defend against the hordes of night-sighted gremlins and other creatures. One must be wary of any underground pool or tight-space, for just about anything could be hiding in the dark abyss.