Lava-punk civilization. Think steampunk, but lava instead of steam.
A city built on the sleeping tarrasque. Sunk into the earth, eternally regenerating, waiting for the end of the world to emerge, its flesh and bone is the basis of a civilization.
Touch on the magical physics that permits the underdark to exist. One idea is that gravity doesn't pull you down, it pulls you towards the element of Earth. This means that sufficiently underground, the outside of every cavern can be walked upon. Alternatively, there is a special down direction, and being above elemental Earth pulls non-elemental Earth towards it; deep underground, rocks float or are pulled towards the edge of caverns, but people don't (as they aren't pure elemental Earth).
Also, the bottom of the world -- go down deep enough, you reach the bottom of the world. If there is a real down, then you risk falling off into the air down there. But with no Earth under you, you don't keep falling, but risk floating off. A civilization could exist hanging off the bottom of the world, mining upwards.
The chained god whose moans make Earthquakes.
A dead god who committed suicide in its grief of killing its mate, on involuntary life support, its heart being pumped by slaves, its blood shining in the darkness. The cult that manages it attempting to bring their ancient god back from the dead.
Parasitic fungi that infect intelligent beings. They don't make them brainless, they just influence their decision making to (a) not disinfecting the fungi, and (b) engage in the occasional irrational behavior to aid the lifecycle of the fungi. This fungi is damaged by exposure to light. Infection might be near universal in the underdark.
An underground "sun" in a large chamber, full of a warped version of a surface fairy-tale forest. It pushes everything away from it, resulting in the entire edge of the chamber being walked upon. The light is actually a psychic hallucination, as is everything you see in it, hiding something else.
The shadowdeep, a portal to the shadowfel mirror of the underdark, or the feydeeps.
The underground library, which gathers all knowledge foul or pristine. Its defences are impressive. Library cards are free for everyone, but most books are restricted, and late fees are extremely steep.
An undersea, an ocean-sized body of water, with fishing communities on its edge. They fish up horrors from the deep and grill them to perfection. About 50% of fishing vessels don't come back. In order to keep population up, everyone going fishing is first mitosis-duplicated using aberrant biotech, and same with the fishing vessels (which are alive). The clones are not perfect. The process is done in the person's sleep the night before hand. A random one is sent out, and if it comes back it is the one that comes back turn. They invite the PCs to go fishing.
