Like the Plane of Faerie, the Plane of Shadow was an echo of the Material Plane, but its geography was altered by an expanding barrier, which contained within it worlds beyond the plane. This barrier was referred to as the Deepmists, an endless gulf of roiling, gray-black vapors that swallowed light and sound. It was an ever-expanding emptiness that stretched in all directions, slowly consuming all lands and life.
Where the Bloomtides compressed the Feywild with crashing surges, the Deepmists stretched the Shadowfell, destabilizing it with unknown emptiness. Demiplanes 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 Deepmists 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 Mortalkind of the Prime Material fared worst of all, as the Deepmists 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 Deepmists often vanished without a trace.