Umbral Plains

The Umbral Plains were a location formerly Argent D'Nur, but was corrupted and incorporated into Hell. There is somekind of association between Umbral Plains and Penumbral Plains.

Background
When the Cosmic Spear cleft a hole through our mortal lands, it tore through pole to pole, from the cold wastes of the Umbral Plains to the fetid swamps of Ironfang.

The Doom Slayer scoured the Umbral Plains seeking vengeance against the dark lords who had wronged him.

The first Project Lazarus Tethering Operation (MTC 2146/351) retrieved a small compendium from the Umbral Plains in the crypt of a destroyed tower of unknown construction (artifact reference PL E2M8 ). This "vade mecum" was mostly illegible, but it did contain one of the rare references to the Aranea Imperatrix - a mystical demon that is still largely unknown.

UAC hsa visited the wide open Umbral Plains during their expeditions to the corrupted Argent D'Nur.

Kadingir Sanctum (the Blood Keep) is a sprawling network of tunnels, catacombs and temples that connect the wasteland Umbral Plains to the Great Steppe. It is believed this area serves as a place of worship and vilification as both hero and enemy can be found buried in tombs across the region.

Behind the scenes
The reference to 'unknown construction' appears to be a reference to the fact that its neither demon nor human and/or sentinel. This appears to be an early reference to the Makr built Citadel (the Tower of Babel).