The Underdark

The Long Walk, The Night Below, The Forest of Stone, The Sunless Desert

It's like the hollow-earth theory is real, and filled with all kinds of monstrosities.

Bards sing of bold heroes who brave the depths of the earth, who plunder the vaults of ancient peoples, exploring and mapping the endless passages and corridors that honeycomb the Underdark.

Although much of this lightless world is empty and devoid of life, it contains terrifying monsters, sprawling alien civilizations, swathes of molds, oozes, slimes, and countless other hazards. But no threat the Underdark conjures compares to the Drow. Their name is a curse, their presence a cancer. They are the despised, the exiled, the shunned—and yet they are powerful, and conquer nearly all who come before them. They are the dark elves, cursed by Corellon Larethian, condemned to spend their days languishing in their own corruption.

Any time someone descends below the surface of they world, they enter the Underdark, but this underworld takes two very different forms.  First is the natural realm as described above, networks of tunnels an caverns formed from stone and soil.  These passages are dark and dangerous, but they're exactly what one would expect to find in an underground realm.  Such passages might be home to carrion cawlers, giant beetles, or clans of kobolds.  But ultimately these mundane caverns follow the laws of nature.

There is another aspect to the Underdark, however:  go deep enough and you find a seemingly endless array of demiplanes, each strang than the last.  When descending into a chasm, explorers have documented findings of everything from a labyrinth inhabited by demons to a real consisting of the guts of a colossal living creature.  Anything is possible in the Underdark, and these "worlds within" are home to all manner of terrors.

The demiplanes of the Underdark are not concretely tied  the world above.  There could be a passage in Hosuth that leads into a disturbing subterranean swamp filled with oozes and slimes.  After traveling what seems to be a few miles, the tunnel may emerge from a different location in Xendra, half a world away from where it began.