An alien city in the Hyades on the shore of the Lake of Hali. The city is dominated by tall black buildings and plagued by strange and unexplainable events. Hastur, both the interdimensional monstrosity and the entropic force, is the unseen master.
Travel to Carcosa is a simple but extraordinarily treacherous affair, and is best left to the brave, the desperate, and the foolish. Residents of the city may offer you maps, but they should be mistrusted: the maps are only correct once, and the residents are usually illusory.
You should, in fact, avoid the people, avoid the buildings, avoid walking around in the streets, and avoid standing in any one place -- all are dangerous activities which can lead to death, madness, dismemberment, and becoming a permanent resident of Carcosa.
Carcosa has been visited by Ambrose Bierce, Robert W. Chambers, and H. P. Lovecraft. They fared no better than anyone else does.
"An Inhabitant of Carcosa" by Ambrose Bierce
"The Yellow Sign" by Robert W. Chambers
Delta Green: Countdown by Dennis Detwiller, Adam Scott Glancy, and John Tynes, pp. 200-213