A world beneath the world, this is a horrible dark place where many of the surface world’s monsters fear to tread. There are many conflicting stories about where the Underdark came from, or who (or what) is actually in charge there… but here are a couple ideas that are generally accepted:
1. Torog, the “one who crawls,” god of torture, lives there (and maybe rules there). There are stories of him occasionally reaching his tentacles and claws up to the surface and grabbing entire cities, so perhaps he’s somehow found a way to break the Gods’ agreement not to directly interact in the world of mortals (which has been the case ever since the Dawn War drew to a close at the beginning of time).
2. Lolth, the drow spider goddess, either lives there and/or keeps most of her people (the Drow) there. It’s unclear who has the most power in the Lolth/Torog equation, but any who’ve looked into it have died horribly.
3. Things that shouldn’t exist do exist in the Underdark. Somewhere in the depths the walls between reality and the Far Realms have broken-down, and many Aberrants roam the halls of the Underdark. Some of these strange alien creatures may have even started to develop settlements & cultures down there.
4. The Underdark changes you. There are fell magics imbued into the very stones of the Underdark, so anyone spending too much time there will most certainly be affected. There are tales of entire dwarven clans who dug too deep and were lost… only to be seen decades later as “something else” that could no longer be called dwarves.
5. The Underdark breaks the rules. Things that normal spelunkers & dungeon-goers know as facts about cave travel, structural integrity, etc. frequently do not apply in the Underdark. There are mosses that have learned to survive by sucking magic from the rocks, deep pits where gravity works differently so that you can walk down the side, ecologies that make little sense to one only trained in the nature of the surface, etc.
6. As if the landscape wasn’t maddening enough in-and-of itself, there are lots of creatures in the Underdark who will drive you crazy so they can feed off of your fear and madness. The Underdark is home to civilizations of Mind Flayers, little parasites that eat brains, giant beasts who drink sanity, that ex you won’t speak about, and a whole host of other things whose sole purpose is to drive you crazy until you die (if you’re lucky).
7. The Feywild and the Shadowfell each have their own Underdarks, but they’re all probably connected in some horrible, horrible way. Just imagining the dreams and ambitions or the nightmares & fears of critters in the Underdark is a terrifying endeavor.
Regions & Echo Dimensions of the Underdark
The Shallows is the label given to the uppermost layer of the Underdark (which is still miles underground). There are Drow, goblins, kobolds, dragons, kuo-toa, and all sorts of other critters here, as well as plenty of strange environmental things, like the reddish areas of “Godsdream” that hold psychic remnants of Torog’s crippled crawling rampage that supposedly formed much of what is now called the Underdark (according to some legends). So far, the Shallows is the only portion of the Underdark the party has visited.