The lost troll capital of Kasmarkin lay sunken beneath the murky depths of the Mannaseh Swamp for centuries, until the Occultists Shakti and Silverstorm suceeded in raising it in the year 298 AF through a complex quest.
Emperor Rakmarkha III, known to history as "the Mad Emperor," was the last troll to lead this empire. It was he who recieved visions of the "Goddess Unborn" (actually Eris, Goddess of Chaos, traveling through time in Her meditations) on how to further his power by summoning the chaotic demon prince Pazuzu to this realm.
In modern times, the city of Kasmarkin may still be risen from the depths, but it sinks again after a time, as the magic fades. One entering the city might find the mummified guards and ghosts of the senators still lurking about, as well as the loyalists-turned-oozes of Lord Taug in his Black Fortress.
On a somewhat less related note, a locked door leading downwards from the city serves as the entrance to the tomb of Overlord Gruul, leader of the Arcanian mercenaries during the Chaos Wars. It was opened only briefly, and the discovery of the Prosperion Bazaar, the temple of Prospero, God of Wealth, and the meeting of Prospero Himself followed shortly thereafter. The key has since been lost.