The Dark Realm - Tartarus
In the Going off Dreams realms, Tartarus is the realm that Eryn first travels to in her perceived dreams. In her reality this is a process for her brain to acknowledge and accept her destiny and altering reality. This is the place where she meets a majority of her allies who join her on her quest.
Within ancient Greek mythology, Tartarus is known as a place where souls were judged after death (Wiki Link). It was also considered to be a deep abyss that would be used as a dungeon for torment and suffering for the punishment of the wicked.
While souls could be there as punishment, Eryn is sent there with a purpose of renewal and awakening. Her purpose is to bring the peace and understanding of the light into the darkest of realms. She's there to acquire her abilities and knowledge so that she can fight the evil within the darkness that lurks to destroy her.
Within the dark realm, light is a scarce source and can only be conjured by those that are for the light. Those that have evil within, or prefer the darkness and wickedness, stay within the dark that scars their souls. Within the first book Going off Drams when light is presented, it is usually a soft light that gives off a romantic glow. For those souls that have cultivated the light, they can produce great fires and such to provide more light.
Tartarus can form to an individuals perceptions. Most of the time the realm consists of deep forests full of lush and dense foliage. To adapt within the realm, and only within the souls that inhabit the area, foliage has been known to adapt and illuminate slightly. Some leaves have been known to demonstrate slight light for souls or being to travel with more ease.
Some of the more dark entities within the realm have adapted as well. Many of the creatures eyes have adapted and can see through the darkness of the realm (which can also make them more sensitive to light sources). Such creatures are usually terrifying and could easily be used for the mythology within the realm, that they are there to torture the souls within that are there as punishment...