The Dark Spirit is a monster kept by the Facility. It is not listed on the whiteboard. It is in the film, but it is never seen. It is briefly seen in the behind the scenes video An Army of Nightmares.
Physical Description & AttributesEdit
Many debate over if it is a Wraith or not, but it is not one because the two wraiths are seen in the movie while this monster is only seen a behind the scenes video. It is a dark cloudy ghostly monster that has some blue on its arms. It is seen lunging at the walls of its cube in the behind the scenes video An Army of Nightmares: Make-Up and Visual Effects.
Maybe a reference to the Dementors from Harry Potter.