Realm of Shadows

The Realm of Shadows is a region of Nevernever where at least some ghouls come from. Briefly mentioned in White Night.

White Night
When the Trailman Twins are abducted from Camp Kaboom by ghouls, Harry questions two prisoners with Lash (Lasciel) interpreting in his head. One of them tells Harry to look in the deep cave above the camp. "The first shaft from the light of the sun. In the stone near it is a way to the Realm of Shadows".