The Sunless Citadel
The fortress that is known as the Sunless Citadel was built to be the home of a cult of dragon worshipers. This cult didn’t just worship any dragon though. It worshiped a beast named Ashardalon.
At some point after Ashardalon passed from the mortal world, the fortress was ‘swallowed up’ by the land. Whether it was built on a sinkhole or if an earthquake caused it to sink (or perhaps an unnatural event), is unknown. What is known is that the magics of the place held it mostly intact as it sank.
Now, the citadel sets at the bottom of a ravine near The Ashen Plains. The place is deep enough in the ravine, that the sun never touches it, hence the name, The Sunless Citadel. Goblins and who knows what else, now inhabit the citadel. The nearest civilized settlement to the site is the town of Oakhurst.