There is such a thing as the Underworld.
In the area near river Shutturga, under the abandoned human city of Halavos, there used to a be a dwarf city. This was centuries ago.
The dwarves happened to dig into a temple to a forgotten god named Thime. Soon, the Sculptor, the city leader, surrendered the city to Thime and started ordering dwarven sacrifices.
There was resistance among the dwarves. A few heroes even managed to take down the Sculptor. He came back a few days later as an undead monstrosity. What remained of the resistance was quickly crushed.
Now, the city is long gone but the temple is still somewhat active, its undead denizen managing to capture and sacrifice a human of a dwarf from time to time.