When you were a kid and had your parents check for monsters under the bed, Vecna is what you had in mind.

The monster was introduced in season four and is described by the streamer as "a former human who mutated into a monster because of exposure to the Upside Down. Vecna lives in the attic of the Creel House, which will be a major setting for this season."

What makes him even more terrifying? He has the same abilities as Eleven—being the original Hawkins Lab test child—and controls everything in the Upside Down, including the Mind Flayer.