Cthundos is a large city in the Underdark. It is currently the home of Lord Kraken.
Eons ago, Cthundos was ruled by the Beholders. However, they were driven out by the Phaerimm, who eventually died out in the area and were replaced by the Illithids. Led by Argus, the Beholders attempted to re-capture their city. They almost succeeded, but they were defeated by Lord Kraken.