Trimval
Called the Red Warrens for the blood spilled in its streets, the main city of Fraib Smaria, with 20000 mostly Tiefling inhabitants. Devils are openly worshiped in its dark red and black temples. The Fallen House, home of the Triarchy of Tyrants, is here.
Interesting locations in Trimval include Sin House, The Bloody Cup, Horticia's Whips, and the Black Cloak Inn.