The Dusk Barrens
The Gloom is a curtain of darkness that originates in the west of the Dusk Barrens. Occasionally, without warning, the Gloom will descend and sweep across the entire Barrens. It seeps underground, envelops mountains, and cover entire forests. It's an endless sea of black.
When a person comes in contact with the Gloom, it swallows them in silence, only to deposit them moments later at the docks of Prosperity Keep, where it finally dissipates. People carried by the Gloom are confused and disoriented for a few moments, with a feeling that has been described as "uneasy, but not unpleasant".
As far as anyone knows, the Gloom has always been part of the Dusk Barrens, predating even the founding of the Empire. Some ancient sources describe it in confusing ways, as if it used to be different in the past.
Two or three generations ago, a small population managed to establish the original Prosperity Keep on the hills west of the river. The current Prosperity Keep resides safely on the eastern bank of the Rush river; It's unclear how the founders of the previous Keep managed to stop or prevent the Gloom.
The Gloom will appear sometime between 2.5 to 3 hours after the beginning of each session. It will descend at a moment of my choosing, grabbing the heroes in the middle of whatever they were doing, to sweep them back to town. This brings the session to an end.
Currently, being swept by the Gloom has no consequences except for being somewhat disoriented for a moment after arriving back at Prosperity Keep.
Currently, the Gloom is unpredictable.
Currently, the Gloom dissipates at the shores of the river Rush.