A Fear's domain is an extension of the Fear, a living dimension that lives as long as it's master lives, and is full of beings and objects related to that Fear. The Fear can, however, change his or her Realm to their will, making it look completely different or changing aspects of it.
The Realm of a Fear has been compared to a beating heart of a Fear, if the realm is destroyed, the Fear dies, and if the Fear dies, the realm does too.
A realm can have any feature a Fear wishes it to have, space, planets, stars, deserts, woods, they can make their realm look like anything they wish it to look like. However, the "default" realm is an infinite, flat world that stretches on forever, with features directly or indirectly related to the Fear.
Realms have several "layers" which require you to go into the realm to access. As you go deeper into a realm, the realm begins to get more and more Eldritch and at some points, breaks the laws of physics.
The main part of the Realm, it contains any buildings, animals, and geographical features that would be found in the realm. Phyics and gravity work normally (or as the Fear desires).
This layer is 50-60 miles under the surface of the realm (4000-5000 in water-based realms) and it is totally dark, pitch black, in this layer, the only way into it is through a cave near the center of the realm. In this layer, the gravity is subject to random changes, never enough to harm the Fear, but enough to slow them down or shock them.
The Core of a realm is a massive cavern, where the laws of physics break down, time slowing or stopped or speeding up at random, ice that is over 5000 degrees centigrade, radioactive material that always gives off energy but never decays and so on. At the very center of the cavern, gaurded by Eldritch Creatures, is a large, colored, floating Orb.
If the Orb is damaged (it has about the strength of glass), then the Realm starts dying. Once the Orb has been shattered, the Realm collapses in on itself and dies, killing the Fear as well.