2. The Mind Flayer
What is the Mind Flayer? Ranking places 1 and 2 is not easy, as the relationship between the two could not yet be fully grasped. Therefore, the Mind Flayer could also be in first place depending on the perspective.
The Mind Flayer is a gigantic entity that resides in the Upside Down and seems to control the collective consciousness there. That’s why it was able to implant fragments of itself into Will, whom it could control.
It is usually depicted as a gigantic spider creature, visually reminiscent of monsters from the genre Cosmic Horror
such as Cthulhu. One can assume that the Mind Flayer existed earlier, but this physical form of the monster was created by Vecna. In Season 4, one can see how it forms the monster from particles after being brought to the Upside Down by El.
The Mind Flayer as a physical creature has not yet come into the human world, but it can exert great influence through open portals. Whether through creatures it controls via the collective consciousness or through fragments of itself, as seen with the Spider Monster.
