Null-Space

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Null-Space, also refered to as N-Space, was another universe. Every living creature in Normal Space had an alternate form that existed in this universe called N-Forms. Upon the death of an individual their dead N-form would leave Null-Space to a higher plane, rather like travelling up a tunnel of light. Their negative emotions were left behind to become monstrous beings.

Occasionally, after trauma or when an individual could not accept their own death, an N-form remained in torment. Because of that, Null-Space was the home of much evil and suffering. N-forms desired souls for sustenance and could escape through flaws in the barrier into our universe, where they relived the last moments of their life as a ghost. Poltergeist activity was common in the presence of an adolescent, though the Doctor dismissed poltergeists as low grade N-forms. (''The Ghosts of N-Space)