Existing like two sides of the same coin, Etherya and the Realm of Dreams exist in the same space between all other realms.
There are three types of citizens that reside in the Realm of Sleep:
Sleepers, those who visit during their time of sleep. They are often unaffected by the environment around them, unable to interact with more permanent citizens of the Realm.
Dreamselves, individuals who have the power to travel to the Realm of Sleep and stay for extended periods of time. These individuals often live separate lives or choose to live solely in the Realm of Dreams. They are often Guardians of the Realm as a caveat to being able to live how they please and influence the Realm itself - though, they're only given a certain amount of space to do so to avoid territory wars.
Terrorgests, the local creatures that devour Nightmares. They are easily influenced by the realm around them and can very quickly become Nightmares themselves if they aren't taken care of.
The River Styx
Separated by the same River Styx, those who enter the Realm of Dreams sometimes cross over into Etherya without knowing it, swept up by the torrent of the River and delivered to the shores at the end to Etherya. Sometimes they're delivered back to the Realm of Sleep, but most of the time it simply means they will be judged accordingly and given their proper judgement.
Only those who are weak in life or close to death are swept up by the River, unless there is malicious involvement in their fate - such as a nightmare that causes great harm to the individual's spirit.
Nightmares and Purification
Everyone experiences nightmares at some point in their lives, no matter the Realm they call home. These are often the influence of negative energies collected from their realm and taken to the Realm of Sleep with them - stress, trauma, anxiety, etc. There are Guardians within the realm tasked to help release these tensions if they are caused by supernatural means, but usually the individual's psyche is built to handle such normal stresses.
Due to the nature of the Realm of Sleep and its proximity to the Afterlife, it was decided long ago that every living being would experience Nightmares so as to help cleanse those in Atonement. In their own way, every living being helps to cleanse the Pool of Renewal so that souls may be reborn with a clean slate. This is why some people feel as if they live different lives in Sleep, or seem to gain fantastic powers - this ability is granted to all so they may continue the fight!
In addition to this, there are Terrorgests that naturally prey on negative energy and cause good dreams in their wake.
Dark Spirits and Leaching
Since the Magical War, individuals of Tenebrae have attempted by any means to pull their darkness and despair into other realms. Because of this, the overflow of negative energy has caused Dark Spirits to form within the Realm of Sleep. This has caused a phenomenon called Leaching, an undesired effect of Dark Spirits entering the dreams of Sleepers and finding their way into other Realms.
[WIP]