Darkness is Bad. We live in a world with light at will and we’ve forgotten how scary the night is. But moreover in a fantasy world it is worse than in ours: it twists and transforms and makes you lost in more ways than just physically.
The places where daylight reach regularly are OK, but the further from the sun you are, the stranger it gets. The darkness is like a strange consciousness with an incognoscible agenda. It moves its pieces against you: their denizens can see in the dark (and lose that ability if they join you, betraying darkness!), the doors close before you and open for its creatures, the traps become weird and jumpy, your careful steps seem to sound amplified, the air thickens making movement heavy and harder (12 feet per minute as movement rate!).
When you spend too much time underground, in the dark, it changes you and makes you part of its files. That’s why dwarves, capable of seeing in the dark, still light their underground cities. Dark dwarves are the prideful cousins who thought they would be stronger than it – they weren’t. Still, dwarves are very resistant to it, being able to remain themselves with only torchlight. The first temptative elven underground cities were grandiose and profusely illuminated, yet they all slowly but without exception declined into dark elves.
Humans are very vulnerable: in just hours they become confused, in a few days their minds start to weaken, in a few weeks they perish or become various kinds of mythical monsters.
It’s not always the same. Sometimes the effect is quicker. Sometimes it’s mild. Sometimes you can spend some days down there, if you rest close to the bonfire, and escape with your self unscathed. The current has some unpredictable weather. But you *feel it*. That’s why most days you need to get out of the dungeon by the end of the game session. The PCs feel the darkness creeping in, soaking their bones. If they can’t make it, darkness takes hold and those characters become lost. If they’re not rescued soon (by new PCs or another party), they’ll disappear forever, turn into dungeon dressing for other group or become one of the dark creatures that can be encountered (recognizable or not. You never know).
Some intelligent and not rarely evil individuals learned to use this to their advantage: Closer to the surface it’s for some time bearable, and its effects can be leveraged to torment prisoners or experiment with strange and weird magicks. Thieves know how to use this weirdness in their favor, and know how to blend into the weird darkness in ways that seem impossible. Sometimes dead bodies rise as undead of they’re left in the dark (like, in a tomb) without being properly sanctified. Some wicked species like beholders or mind flayers are even reinforced in it. In any case, the further deep and way from the day you go, the worse it is (hence dungeon levels).
Maybe the underground craves treasure, or wants back the gold and gems excised from it, so it makes its creatures rob and hoard them. Maybe that’s why it hates adventurers – adventurers steal back what she finally regained. Maybe it doesn’t hate dwarves so strongly because they just work the precious metals and crystals but don’t take them away from it, keeping them close underground.
Sometimes it’s the underground who extends their influence outside, seducing mortal minds into it with whispering voices or the dream of riches and power, bringing kings, scholars and fools alike into it like a spider tending its web.