" Dark Specter** - A frightening variety of Type Two ghost that manifests as a moving patch of darkness. Sometimes the apparition at the center of the darkness is dimly visible; at other times the black cloud is fluid and formless, perhaps shrinking to the size of a pulsing heart, or expanding at speed to engulf a room. "

― Jonathan Stroud , The Screaming Staircase (Lockwood & Co., #1)