Jack

Jack does no damage at all; its only job is to scare you, and it does that by making a loud noise when you see it. Then it vanishes, usually in about a second. The wiki lists it as a passive entity that lives in The Hotel and The Mines, and it is one of the rarest ones: a black-and-white head with hollow eyes and a smile that shows only its top teeth.

Two ways it can appear

Each time you try to hide in a Hotel closet or a Mines locker there is a 1 in 200 chance (0.5%) that Jack is already inside. It will not let you in, the doors shut abruptly and the nearby lights flicker. If you open the spot again it has most likely vanished and you can hide normally. The second form is far rarer, roughly 1 in 10,000 to 1 in 20,000 when you open a door: the room turns dark red with moving black dots, and rain outside a window stays red for good.

What to remember

Jack switches off during Rush and Ambush attacks, but not during Blitz, Figure sections, glitched entities or in Cringle's Workshop, so hide a little earlier there. If it ruins a hide and you die to Rush or Ambush, the death message says you could not hide. In very large rooms the hallway form can cause heavy lag on low-end devices.