The Sleepers' March

The Sleepers' March is the name given to the Underdark under the Charred Veld after the Underdark War occurred by the drow refugees who crossed it to reach Chel'issera from Ezzebek. Before that, it was simply Underdark, Forbidden Territories, or The Elder Dark, for this is where the forgotten deep ones lurk. Far, far below the elves above, driven away by the elven druids and protectors and fended from the cities by the drow and duergar, bizarre elder evils lurk, preying on each other and any who happen by. This is where illithid cities can be found. Aboleth pools. Colonies of beholders. Deep dragon lairs. No one ventures deep into the Elder Dark unless they're foolhardy, brave, or incredibly desperate. Even remaining as close to the surface as possible is fraught with danger, and the deeper you go, the more prevalent and stronger the dangers become.

And even worse, with the lack of elven forces pushing from above, and with the drow tied up with their own politics, some of these elder evils are starting to send out feelers into higher regions of the Underdark.