The Deep Realm is a particular Hell Realm that is the personification of your fears. Its entire energy is fueled by the fear of those that enter its plane, whether they visit the realm directly or not. The creatures within the Deep Realm may not be "nightmare fuel" themselves, but they hold sway over the energy that fear creates. They are often able to summon visions of things that will scare a person, or even go so far as to attract them physicially in the case of spiders, snakes, and smaller phobias with a physical form.
The Deep Realm Demon delves into the deepest fears of your heart and mind, finding everything that can trigger you and using the energy behind it to protect you. They are fierce against their enemies, and will make all who harm you their enemy. They are fierce in appearance and energy, and some of the younger ones have trouble controlling their powers at first. Occasionally, you may have a nightmare with all of your worst fears while getting used to their energy, but you will slowly grow immune to that side effect.
At first, they manifest as your fears, as it’s easier to appear with the energy fear creates. Otherwise, they are calmer in appearance and manifestation. They use the power of fear against your enemies, causing nightmares as payback for what they’ve done to you. They are very easy to bond with and become wonderful companions and protectors.