Nightmares are a demihuman subspecies of Centaur that can enter the dreams of sleeping individuals and create nightmares, although the exact reason why Nightmares must give other people nightmares, except from that it is part of their culture, isn't explained. If a individual is awake, a Nightmare can also use hypnosis to put the person into sleep, although this skill must be learned as hypnosis is not an innate ability of the Nightmares.