Dreams

Dreams are the visions one has when they are asleep or unconcious. There are many legends and stories of the importance of dreams as well as the dangers. There are stories of prophecy from dreams as well as those who have died in their dreams leading to their death in the Material Plane.
Those more educated may know of how almost every creature dreams and when they do so it creates a "dream-self" which is like their soul that leaves the Material Plane and goes through the Border Ethereal into the "Wall of Color" or aka the Ethereal Curtain that exists between it and the Deep Ethereal. The dreamer creates a demiplane of sorts inside of the Plane of Dreams inside this mystical and magical place, and that can drift around even going into the darker Nightmare Realms.
Many believe the magic of the Feywild and the Plane of Dreams is connected, as well as the Shadowfell and the Plane of Nightmares.