The ability to Dreamwalk was first conceived by the Teshial. It consists of appearance and conceal you from others around you. It also deals with developing consciousness in dreams. Prior to the Teshial, dreams were considered very mysterious and uncontrollable. The mystical elven caste discovered what is deemed the "Curtains of Awareness," which is the thin barrier between the unconscious and conscious Dream Realm. When the elves became aware of their dreams, they were able to interpret them and make predictions accordingly, as well as wander into others' dreams.
Dreamwalking is a very complex form of Dream Magic. It allows you to walk consciously amongst the Dream Realm, and even alter it. In order to walk, you must have a clear and focused mind. Thoughts must be controlled, otherwise your dream could end up consuming you. Free-wandering of the Dream Realm is unadvised and dangerous, so the elders of the most recent dreamers, the Nightmare Knights, developed specific paths through the world of the dream which would lead to a specific destination. These were called Dreampaths. All of the paths are connected to the Chamber of Dreams, which acts as a controlled entrance/exit point to the Dream Realm. The Nightmare Knights became renowned for using their knowledge of dreams in order to fight impending doom. When the Ruthless Seven eventually invaded their dreams, their forces took over nearly all of the paths and halls in the realm that the knights had used. Using their own dreams against them, the demons were able to seek out and destroy the now separated knights.
However ... it is said that the last Dream Master, who had previously taught the knights in the art of Dreamwalking, still exists somewhere in the Dream Realm, but since the realm is endless, it is impossible to find him ... unless you have a path there.
The Void that was created in the locked closet in the Necromant House was a failed attempt to create a path to the Pits of Inferno, which accidentily created an entrance to a remote part of the Dream Realm. It serves no useful purpose (except for a quick escape from the Plains of Havoc).