Cthorgo is a starless, though not light-less sea of Imgangreth.
Journals from the Gallant Nine, an adventuring group comprised of nothing but Mîmêk, tell of encounters with insane aboleth, underwater demons and water archon in the area around the half-submerged ruins of Titaram. One of the group, notable for his vast knowledge of the realm, stated that the unusual architecture of the city was a mix of Lith-Crillion and aboleth. Another journal entry details a conversation with an aboleth - 'the slimy creature said that thousands of years ago the aboleth were slaves of the Lith-Crillion. They had the aboleth tear down their old cities and rebuild them to the specification of their slave masters'.
The Great Tradeway crosses the sea along a series of islands. Some of these are linked by bridges, the others require the use of ferries or swimming. The center island holds the ruin Belephant-Elebros. This city fell in the Tradeway Wars, yet its Vampire Queen, now with few subjects, still claims to rule the ruin and the sea lanes about it. Most travelers of the Great Tradway, bypass this area by taking wide arcs around the haunted ruins and its blood-thirsty ruler.
Cthorgo provides sustenance for many peoples and sees much trade. Some of the Underdark trade that goes across this sea is taken to passages leading to the surface where it goes to holds in the Tormoran Federation, the Orchish Empire, Crimson Eye, and dozens of smaller groups.
Cthorgo can be reached directly from the surface by shafts that plummet from the surface area Knur Slagming to the sea's roof. This is a long journey requiring a lengthy climb of up to two miles, but one normally undertaken by flying. A bard of the Marching Twenty wrote a poem about these shafts, calling them Phalgas's Eyelids. Books on that party of adventurers say that they used magic to fly down into one of Phalgas's Eyelids and explore areas around Cthorgo. They say that even after they went two miles down the shaft, there was another mile to fly till they reached the dark waters of an Underdark sea.