Midnight Visitors are denizens employed in service as the troops of Monday's Dusk, and as such they guard the Lower House.
Like Commissionaire Sergeants and Monday's Dawn's Inspectors, the Midnight Visitors are Denizens created by the Architect as opposed to being made in the Far Reaches by Grim Tuesday. They wear black, including top hats with long black ribbons tied around them. According to Suzy Blue, they wield Nightmare Whips and Night Gloves, though she does not elaborate to say what effect these items have when used.
When Arthur became the new Monday, the then Monday's Dusk requested that many of his Midnight Visitors be given a change of post, as they grew weary of wearing black.