Location of the nightmare chase at the beginning of Chapter 1. Full of yellow fog. Gray brick walls tower overhead above where the fog thins. The walls sag in at uncomfortable angles. Distant buildings seem domed or arched, empty, or not. Streets of broken cobblestone below, and an oily purple moon bruises the sky above.
Ray had a brief vision of this city during his nightmare in Chapter 6.
They discovered later that same chapter that this city was being depicted in charcoal and yellow chalk on the walls of houses in Thrushmoor.
In Chapter 43, the party recalled the name of the city: Carcosa, and they thought of it as where Hastur either lives or rules or is imprisoned, and the place to where Xhamen-Dor drags the worlds it conquers.
During the botched “Great Harvest” ritual in Chapter 51, Carcosa was briefly viewed as co-existing with Thrushmoor. In the Carcosa-version of Thrushmoor, Iris Hill was absent, replaced by a windowless tower with battlements. Upon seeing the city, all (who hadn’t already) immediately knew its name.
First Reference: Chapter 1
Other Notable References: Chapter 6, Chapter 22, Chapter 39, Chapter 43, Chapter 51