In the far west, under a permanent cloud, sits the Obsidian Castle. Twice it has protected some powerful beings bid of domination of the world, twice is has been foiled. But the Castle is patient, and is already nuturing the third, who has already begun his march.
The Castle is made of jet black obsidian, each block is exactly the same size, mortared to the next with a dull brown film, the blood of the victims sacrificed to build it. Enchantment runs through the entire structure, oridinary weapons can make no mark upon the walls. The castle is black - gloomy, and horrific. Light cannot travel far within it - absorbed by the walls. The floors within are pure black ebony, with no trace of light or color. It's hard to breathe in the castle, though character never seem to run out of air.
The castle actively protects the Dark Lord. It has a nearly infinite supply of glassy obsidian or ebony or black granite guardians. Gargoyles guard the upper heights, razor-winged obsidian bats range the great halls, the moat has no water but is filled with delicately balanced sheets of razor-sharp glass that would instantly shred anyone who fell within, even in armor, for the points would find every gap and pierce the body within.
The Castle is the home and last redoubt of the Dark Lord. Your characters must raise an army to defeat his orcs, ogres and trolls. They must forge a treaty with the beings of the light and air - the eagles, the ki-rin - to provide protection and cover against the Dark Lord's leather-winged reptilian flyers. But the army is mere diversion - to get the players into the Castle.
Deep inside the bowels of the Castle is a room perhaps 100 feet wide and nearly as high, and paved with gold. The walls are bright polished marble, hung with cloth-of-gold and studded with endless tiny gems. The ceiling has an enormous crystal chandelier, whose bright glow is nearly eclipsed by the six-foot-diameter gem on a low dais in the middle of the room. The gem is a composite, made of thousands of smaller gems, from fist-sized to tiny grains, of every shape and kind. They are packed into a great sphere, facet-to-facet, edge-to-edge, and the sphere is alive with light of every color in the spectrum. Bolts of light flash from point to point within - tiny dots in many colors swirl about inside. The evil spirit of the Castle - its "brain" - dwells within. No living being has ever entered this room - or even knows it exists, but until the gem is destroyed, the Obsidian Castle will always rise again, and new Dark Lords will threaten the world...
Of course, you'll need to work out a lot of details, but this idea should be good for three or four campaigns before they figure out that is isn't "just another Dark Lord" but the Castle itself that is the real enemy, and that destroying it is merely a temporary setback. You'll need to decide who built it, and why, and when. You'll need more monsters in the "broken glass" idiom - many people are afraid of broken glass, it's a powerful symbol. Perhaps the Castle is lit with black light torches - you can see, though all is black and dark, and the flames rime the walls with frost and burn like frostbite...