(Inspired by a role playing game I review – The Nocturne by BTRC)
By Church Doctrine, there is the Materia (The Material or Worldly Existence) and the Apocrypha (The Reams of Theurgy and Psy energy) and the Caelum (Heaven). This is the Holy Trinity of Realms.
However, hidden in this world is a world where Magic and Psychic power meets, a world of Dreams and the Unreal. This is called the Nocturnus. Sentient beings like humans can only touch on it through Dreams and Astral Projection.
In this dimension, alien near-human beings called Others exist. They were Materia beings that ascended into the Nocturnus.
Another way the Nocturnus crossed over into the Materia is the Shadow Realm and the Shadow Demons. Some Shadows act as portals from these brings and allow them to teleport from one shadow to another.