The Gloomshrouds, sometimes called the "Deepmists," were an endless gulf of roiling, gray-black vapors that stretched continuously across the Umbral Realms in all directions. They were an ever-expanding emptiness, slowly consuming all lands and life while swallowing all light and sound. Like the Plane of Faerie, the Plane of Shadow was an echo of the Prime Material, but its geography was altered by this expanding barrier, which contained within it Demirealms beyond the plane.
Where the Bloomtides compressed the Feywild with crashing surges, the Gloomshrouds stretched the Shadowfell, destabilizing it with unknown emptiness. Demirealms were contained within the mists, and over time, the Shadowfell's territory was dilated as these domains drifted. Legends told of Dark Powers dwelling within, unseen but ever-present, bending the vapors to their will and watching from behind the curtain of fog.
For many powerful creatures with domains on the Shadowfell, the Gloomshrouds were a genuine existential threat. The domains of Darklords were consumed and lost to the mists, further worsening the isolation, loneliness, and despair inherent to the plane. The Mortalborn of the Prime Material fared worst of all, as the Gloomshrouds could spill through the planes where Leylines were close. The mists sapped vitality, paralyzed flesh, and were generally poisonous to organic beings; those who strayed too far into the Gloomshrouds often vanished without a trace.