The Realm of Madness is a mystic realm where one's greatest fears take on a tangible form.
It was set in the Archipelago of Anguish and Redemption (also known as the "Splinter Realms"), and was adjacent to Nightmare's realm, beyond both it and the Dream Dimension, and whose denizens are natural enemies to Nightmare. It is usually accessed through magic. Though it lies in some sense beyond the Dream Dimension, it can be accessed directly.
The realm was ruled by Dusk, who commanded legions of demons such as Delusion, Dementia, Fetish, Megalomania, Necromancer, Neurotica, Obsession, Paranoia, Phobia and Psychosis, though some of them served Nightmare as well.