Catacombs are religious sites composed of a series of interconnected underground tombs. Often they house the skeletal remains of monks or other important figures for a religion. Fictional catacombs are often used in sword and sorcery worlds, or worlds with a slight horror element. Catacombs may feature walls of skulls, candles, altars, shrines, crypts, and relics. In game-oriented worlds they may function as dungeons. Catacombs are home to living skeletons, monks, and spirits.

Catacombs are often underground, labyrinthian mazes. Those that do not know the space or do not possess a map may become easy lost in their depths. When designing fictional catacombs, consider fostering a sense of claustrophobia. These narrow passageways may be jarringly interrupted by vast chambers containing altars or temples dedicated to a religion or god.