Underdark

The Underdark Posited as a Pocket Plane Accessible From Many Points on Summer's Tale
The Underdark is a semi-mythical realm that is always "just beneath the surface" of the experienced material reality. What this means, or how i(or where) it exists is a point of question. All that is known is told by those that have descended to its world and come back alive to tell the tale. It seems that the Underdark is accessed as a place "underneath," usually meaning some kind of hole or portal down into a strange, dark, often creepy world. The world has been described as like a dungeon, a maze, layers to a castle, or even a forest or mountainous region. Many other descriptions have been given, so that there is confusion among those who would understand.
It seems that the Underdark is not always terribly dangerous, but it is a world of creepiness, a world of unexpected creatures, landscapes, or horrors. Those that have "gone under" to study such things have met with mixes success at best, and a horrible demise in the worst case.
THe Underdark is not truly a separate plane, but it is a place on (in?) the Material Plane that behaves a little like an evil-aligned place. It is dark, and its minions are exclusively evil (or neutral-hostile, so rarely good). It is really like its own world, very rarely interacting with the light. People speak of it as being “beneath,” but its qualities are hard to pin down that way. It is its own pocket, and it hasn’t been confirmed or denied that the different Underdark pockets connect with each other. They do have more or less the same properties, although there are exceptions...
There are portals from the Underdark to various other Pocket Planes or Evil-aligned Planes. Some posit that the Underdark is itself actually a huge collection of Pocket Planes that are somehow linked--in them if not physically.
A series of tunnels and caverns and rooms and even strongholds runs “beneath” (often far-beneath) the surface of the material plane, in a network of darkness and shadow. Strange creatures populate the underdark, some of them known above, and some of them purely unique.
From Dreamsgate, Hela Goddess of Death has come to dominate much of the Underdark, and she uses it as a dungeon, palace, and breeding ground for her minions. Her plan is to one day (soon) open up an entrance in the Shadowfell ( in the Never Forest specifically, on Dreamsgate) and flood the surface with her terrible hordes.
The Underdark beneath the forests and jungles of Neverland, on Dreamsgate, are particularly nasty, cruel, dangerous, and evil.
The Underdark portals / pockets crisscrosses among the nodes of negative power, of which there are few. The realm is used by creatures of shade, sometimes to travel and sometimes to dwell out their lives. The Underdark is “below,” and it rarely influences life above. Some notable entrances are rumored in Dreamsgate: in the Nameless Mountains, Darkest Mountains among the Drow, and the Never Forest, for example.