The Hat Man is a type of shadow person encounter often associated with sleep paralysis and overuse of benadryl. Like other shadow people the hatman appears like a shadow but more specifically like some sort of void, usually standing in the middle of the room when you wake up in the middle of the night.
Descriptions of the Hat Man date back to at least the mid to late 2000s.
The Hat Man has since appeared in media such as the visual novel Class of ’09 where he appears to one of the characters in the middle of the night.