The Night Doctor was a plague doctor in Edinburgh in 1645. He would only work at night.
He would visit those who were sick with the plague when it was soon to be their time to die. He would visit once and then leave, this signalled that the patient would soon die. When he returned the patient had usually passed away they would be carried away to an unknown location by the Night Doctor.
It was later discovered that the Night Doctor was Annabelle Rae's plague Doctor suit being animated by the Psycholops. (PROSE: Plague City)