The Dark Fortress of Desolate Darkness resides in the Nightmare Realm, where the owner's own nightmares and fears come to life. To prevent others from seeing the full contents of said things, Brespawn has limited it to things she's comfortable with sharing. Fears such as: heights, losing the ability to walk, fire, etc.

The fortress itself is inaccessible but there's a resting area to the side where weather gathering experiments take place.