Duskmourn is a modern-looking world, including electronics like television sets, mechanical devices like chainsaws, and 80's style fashion like sneakers.^[5] Its original name now long forgotten, the plane once had varied geography, including mountains, seas, coasts, and cities.^[6] It was composed of three parts: a physical realm and two immaterial realms inhabited by spirits and demons respectively.^[3] The plane had two suns, one greater and one lesser, that rose in the west and were consumed by the House.^[7][8] As far as anyone knows, there is nothing outside the House anymore.

At least three generations before the Omenpath Era, the entire plane was engulfed in an endless haunted mansion filled with horrors.^[7] The House is no ordinary building; it's a living, plane-spanning entity that thrives on the fear of its inhabitants. The House is the world, and the world is the House.^[6] Its rooms are constantly shifting, changing more significantly when not observed.^[1][9] Sometimes there seem to be outside environments, but there is always a door or window leading to the next room. The moths that are the symbol of Valgavoth appear everywhere. Each moth has multiple eyespots on their wings, functioning as the eyes of the house. If you can see a moth, the house can see you.

Harrowings occur whenever Valgavoth molts out of his latest form and into a new body. The surge of malevolence that comes with this rebirth extrudes a fresh wave of cellarspawn into the House. It also sends out a psychic ripple that incites cultists, razorkin, and other monstrous entities to increased viciousness, bloodlust, and paranoia. Harrowings typically only last a few days. Harrowings are offset by Quiescences, which occur right before Valgavoth molts. With all his attention turned inwards to focus on his new growth, the House falls into a temporary lull that's as close to safety as it ever gets. Quiescences typically last a few weeks, giving survivors much-needed time to recuperate and prepare themselves for the upcoming Harrowing.