The Umbral Plains are a location in Hell.
Background
The Umbral Plains were one of the regions of Argent D'Nur pulled entirely into Hell following the Argenta Civil War. Formerly, they were located at one of the planet's poles, and had an arctic climate. In Hell, this area borders the Great Steppe, connected by a labyrinth of tunnels and catacombs collectively known as Kadingir Sanctum.
The Umbral Plains were the site of many of the Doom Slayer's battles against the demons, and are described as a vast wasteland. Later they were the subject of a number of UAC expeditions which bought back artifacts, including a severely damaged tome called the Vade Mechum found under a destroyed tower. This tome, though mostly illegible, included references to a fearsome demon known as Aranea Imperatix.