Imagine kingdoms and cities -- some governed by goblins, some by dwarves, and some by gnomes. Imagine tunnels. Some will cover great distances horizontally, like roads connecting cities on the surface. A few will be expansive thoroughfares large enough and smooth enough for wagons of wares to be hauled by larged-eyed, underground beasts. And still other tunnels will lead down deeper into the Underdark, where less civil creatures have their own enclaves.

The Underdark is a world with symbiotic relationships. Cities trade with each other. Some trade with the surface and even get caught up with the conspiracies of surface politics. Others are more predatory -- raiding and attacking rather than growing and making their own. Imagine fortifications. Chokepoints. Where are there lost kingdoms now full of vile creatures; those who just want to be left alone; and secret societies whose activities were too taboo for the surface world? Imagine a cult of humans committed to breeding with drow elves. What are their foul, long-term plans for the surface when their offspring mature?

It falls on you, the Dungeon Master, to help place your players in this bizarre world. Is there a constant chilling breeze on their skin? What flickers in the shadows of their torch light? What do they hear? Perhaps a constant sound of running water. Strange clicking or crashing sounds. Low rumbles that shake the earth and wake them from their fitful sleeping. Or perhaps booming eruptions that keep them from sleeping at all.

What do they smell?

Taste on the air?

Think about “weather” systems as they move in the Underdark.

Near large underground rivers, the air might be more humid. If they travel deeper, the air might get more hot and smell and taste of sulfur. What might a forest of glowing mushrooms smell like?

Battle also becomes challenging and exciting in the underdark. Some passages might be so smooth, only creatures who can levitate can get across them. Narrow bridges make for unique combat situations. As your players swing or lunge at opponents, they might also need to make “balance” rolls based on their dexterity. A poor roll might mean they plunge into a spike field of stalagmites below.  Retreat is a less likely option both from underdark combat and the underdark world -- adding a sense of urgency and doom. A wall of spikes is called a ladder in the underdark and might be your players’ only choice to ascend from danger. Winged creatures fly into their faces, and pale insects climb from the crevices of the walls across their fingers.

You can enhance the atmosphere of your roleplaying space to enhance your players’ Underdark imaginations. Light your roleplaying room with candles or at least dim the lights. Black lights and glowing paint can further add to the alien nature of the Underdark. Find anything that glows in the dark. Turn up the air conditioning. Or, even run a vaporizer to give the room a misty, underground sensation.

With forethought and a commitment to its challenges, you’ll be able to create an incredible Underdark campaign for your players. You might very well be giving them and yourself an experience unlike any roleplaying experience before.