Nightmare is a ghost you won't want around, it's said to roam around looking for people to imprison them in the underworld.
Strengths: Nightmare will turn off all lights and close all doors (you can open them, except the main one) and emit a local sound, whoever listens will lose 50% of their sanity.
Weaknesses: Nightmare does not like light, so lighting a candle and placing it on the ground will prevent him from passing through the place, until he extinguishes it.