An Alternate Underdark (The Underdeep)

The Underdark.  A massive series of caverns cobwebbed under the entire globe, many large enough to fit whole cities, while others can be a maze of twisting stone passages that make it easy to get lost.  Home to the infamous Drow, Durgeur, Illithids, etc.

It’s become a pretty well known segment of the typical D&D world, and for understandable reason. It basically takes everything popular about crawling around an underground lair where everything wants to kill you and ramps it up to 11.

Players like the sense of danger, DMs like the fact that it’s somewhat easier to build and direct players in certain directions with, and both like the relative simplicity.

But I think we can do better.

Why not take the same basic stuff from the Underdark, and put it on the bottom of the ocean instead?

I don’t mean a series of caverns in the ocean floor, or a giant bubble underwater.  I mean take all the creatures, monsters, etc, make them Aquatic, and have them all live on the bottom of the ocean, in Davy Jones’ Locker.

Underwater Shot of Pile of Fish during Daytime

The Underdeep

Hear me out.

  • It ranges from dark to inky black on the lower levels of the sea, even with a light (which attracts the wildlife, so you need to weigh your options).
  • Aside from the palette-swapped clones of surface races (Drow, I’m looking at you) the majority of the things found in the Underdark would seem right at home on the ocean floor as well.  Heck, it’d make it even easier to use Aboleths and the Kuo-toa.  And Beholders and Mind Flayers already look like they belong down there.  Cloakers could be a kind of Manta Ray, etc.
  • The life that already exists down there is terrifying enough as it is, not to mention all the sailor’s tales of sea monsters you could use.
  • It requires specialized magic or technology for normal people just to SURVIVE down there, (which puts a whole new perspective on the Beholder’s antimagic field) as well as leaving the exact method of survival up to the DM to tweak as they like to adjust the tension or feeling they want to evoke.  (You could have everything be relatively stable, or based on an item that also attracts attention so the PCs have to defend it, or anything else you can come up with)
  • It’s an environment that already exists in almost every setting you’ve seen, was likely not being used very much, and you don’t have to wonder why the surface doesn’t just collapse already from being hollowed out with so many Underdark caverns.
  • It even has a multitude of different environments and ecosystems within it, from bright and vibrant kelp forests and coral reefs to brine lakes (yes, there are lakes underwater, and they kill things because why not), to underwater mountains, sinkholes, valleys, canyons, and vast empty plains.
  • It’s a simple matter to reskin some already existing monster stats into things like a Giant Anglerfish, but the completely new look will definitely throw your players off and make it feel brand new again.
  • It’s big.  I don’t think most people really conceive of just how mind-bogglingly HUGE the ocean is.  This, along with natural buoyancy, makes it much more believable and realistic to have giants of one kind or another.  Creatures the size of a small mountain who can stand straight up and still not brush the surface.  And you know all that stuff you’ve packed into a handful of continents in your world?  Odds are you’ve got even more space to play with in those oceans of yours.

Heck, the very floor of the ocean is called the Abyssal Plain.  How’s that for an inspiring name?

Think about it, your players walking along the sea floor, literal miles of empty water between them and any breathable air, save the small magical or technological trinket each of them wears.  If those devices fail, they’d die before they even knew anything was wrong, the sheer pressure of the deep crushing them like a paper bag that had a house dropped on it.  They’ve elected to use torches/flashlights, and aside from the curious sharks who come to look at them now and then before swimming off, it seems silent.

The darkness stretches in all directions.

Then, suddenly, they see the bottom end of a huge tail fin, twice the size of a building, slowly glide into view and glide out again.

And once more, all is quiet.

black-and-white, dive, diver

What’s that?  You like the Drow?  Simple, replace spiders with giant long-legged crabs (like the Japanese Spider Crab), and give the culture to Mermaids or Sirens, luring sailors to their doom and wrecking ships to steal away a fresh supply of prisoners and/or food.

Or just have some species of spider live in the ocean.  There’s already spiders and beetles that live under freshwater.  You could even have the Umber Hulk be a version of those for salt water.  Oozes fit pretty well underwater, too, and could even be adapted as some sort of plant or algal slime.

With the Underdeep, you replace the cramped cave walls of the Underdark for something far more terrifying: vast black emptiness, where death can approach from literally any direction, even the very ground you trod on or swim over.

This also gives you the perfect opportunity to try out some Cthulhu lore if you’re so inclined.

The players are at a disadvantage if they go in with their normal door-kicking, club-swinging routine, on top of being completely out of their element anyways.  Everything they find there has an actual home field advantage.  And they know it.  Creatures ten times their size can easily and silently sneak up on them, shipwrecks and sunken cities could have old treasures for the taking, and the nameless cities of the Aboleth and Mind Flayers could wage war or trade goods and slaves indifferent to the world above.

You don’t even need to involve a sentient race of horrors.  The natural world down there is horrifying enough!

Plus, how better to make the PCs feel vulnerable than to make them feel so very, very alone?

So give the Underdeep a chance!  See how your players handle a REAL challenge.


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