Cthulhu City (Trail of Cthulhu Supp. Hardback) - Rules of Play says:

Cthulhu City is a setting for Trail of Cthulhu, usable for a full campaign in its own right or as a nightmarish intrusion into an existing game. The Investigators find themselves in a strange, corrupted Arkham, a ghastly metropolis. People – humans – live in the city, and seem bizarrely normal on first encounter – their concerns are the same mundane, day-to-day passions and trials of anyone in the modern world – but scratch the surface, and the Mythos spills forth. Motorcars drive down streets lined with sullen-eyed basalt cyclopean buildings raised by no human hand; at night, loathsome and titanic shapes move behind the clouds. It’s a city where priests masked with yellow silk proclaim the majesty of God from the churches; a city occupied by alien powers. The old-money families have names like Marsh and Whately and Curwen, and the worst crime imaginable is defying the will of the living gods.