Carcosa is often desribed to have:
- Black stars in the sky on a bright sky
- Twin black stars
- The Lake of Hali
- Black soaring towers
- Wet winds
In the Pathfinder universe, Carcosa is a highly magical planet which contains a living city that is also called Carcoa. It's the home of Hastur, who resides in the monstrous Lake of Hali. When the light from Carcosa's star shines on another world's night sky, Hastur's power increases on that world. Carcosa is continusly expanding, absorbing the societies and cities of other worlds. Serving as the fuel for Hastur's eventual ascenscion into the ranks of the Outer Gods. At which time he will consume and absorb Carcosa and its inhabitants. The city of Carcosa is a vast patchwork of the cities which it absorbed over the eons, each individual patchwork is a nexus, which may retain some of the properties of their original worlds. Some of the known nexues include Neruzavin (flying polyp city), Bohlvari (shoggoth inhabited elder thing city), a technologically advanced city of skyscrapers (inhabited by robots and aliens), and an underwater city (inhabited by the undead).
- First mentioned in Ambrose Bierce short story An Inhabitant of Carcosa, and later expanded in The King in Yellow by Robert W. Chambers, just like Hastur.