The Dark Spires is a tall tall tower that stands on the very peak of the Brogendenstein Mountains. It's tall, onyx outside belies its hot, warm interior, and just inside, an Idol to the god Kossuth can be found. The first few levels have a general tower appearance, but as you go lower, you can see that it starts diverging into a long lost mine, now overrun by Fire Giants. Its lowest layer is filled with hot lava, and contains the Throne of the Fire Giant Royalty.
The Dark Spires were not always so dark, nor did they always only go "down". Once, long ago, they were held by the Dwarves of Brogendenstein, who used them in their mining operations, in order to go deep into the mountain, and excavate what was within. As is typical for Dwarves, they eventually went too deep, and opened a passage into the Underdark. From within came the Duergar with their tamed Deep Dragons, and the Dwarves were forced to retreat from their mountain abode, and were forced all the way back to the coast. The Duergar continued to expand the mines, and kept a tight rein on their most precious artifact, the Gong of Rifts, and the accompanying Rifthammer.
Eventually, the Duergar themselves mined too deep, causing the lower levels to flood over with molten lava. This opened the way for the Fire Giants to swoop into their new favored terrain, and like the Brogendenstein Dwarves before them, they were slain and/or driven away. Today, the Dark Spires still stands as a testament to Giant power, inhabited inside by Fire Giants and outside by Frost Giants.