The Underworld Barrier

In a time when humanity was just spreading out across Vall Rusk, the Underworld was already a major threat to the races of the Surface. The races and nations of the Underworld conducted frequent raids upon every race on the surface for loot and slaves, some with a dedication to tormenting their races' surface equivalent.
Once it became bad enough that other threats started taking advantage of the chaos and drain on the Surface races these raids caused, the Alsmage stepped in and erected a barrier on the Underworld.
At a certain depth, a colorful, horizontal wall blocks the way for all those who were born in the Underworld, but allows those who were born above it to come and go as they please.