Carcosa is a term found in many works of fiction such as those written by Ambrose Bierce and Robert W. Chambers, who is probably mostly associated with the name. It usually denotes a place or a city and usually is related to The King in Yellow - though what exactly he is may vary from story to story.
On True Detective Edit
Carcosa seems to be the name given to the old stone ruins in the Louisiana bayou area which are shown on Season 1's last episode. It is considered a place of worship in which macabre rituals were performed, often including child sacrifice and sexual abuse.
It has also been speculated that Carcosa is actually the name of the mystical vortex found in the main chamber of the ruins, though in contradiction others pointed out that this could have been just a hallucination by Rust.
On other media Edit
The term is often associated with H. P. Lovecraft despite him not being it's creator. Lovecraft's fictional universe and themes have been used, with his permission by many other authors creating what is know as the Cthulhu Mythos.
The direct reference to Carcosa and The Yellow King make True Detective, beyond any doubts a part of the Cthulhu Mythos - not only formally but also by having many elements in common with these stories such as satanic (or alien) evil forces, heavily mystical connotations, a pessimistic worldview, inbred mixed sea-monster-and-human creatures (in this case bayou instead of sea-related) - not to mention the narrative format which is typical of this sort of horror story, with heavy emphasis on research and information gathering and later fast-paced, intense short action-horror scenes (like the attack on the meth lab and final showdown on Carcosa on season 1).
It could be said one of the biggest achievements of the series' first season was to create an adaptation of what would be a classic Cthulhu Mythos tale into mainstream media, which is usually a hard thing to do given the stories are very slow paced for most of the time. True Detective managed to do that largely by following the conventions of a criminal thriller series with hints of the macabre but eventually evolving to a fully-fledged horror story.
- Wikipedia entry on Carcosa, with some information about True Detective alongside a list of many other authors and media who make use of the term, including world-class authors Neil Gaiman and G. R. R. Martin.