Derenath was the capital city of Estorium. Before the Illithid War, the city was one of the greatest in the world. In the center of the city was the majestic Crystalline Palace. It was one of the first cities hit when the mind flayers invaded, though, and now lies in ruins.
Seven hundred years after the city's destruction, the Brightstone Heroes were transported there while attempting to return to Silverstone. They defeated a number of grell that had made their home in the ruins, and eventually discovered one of the keys to Doth'Mordun that had been hidden there.