The Mind Flayer

The Mind Flayer, the particles, and the Shadow Monster were all names used to describe a shadowy entity originating from the Abyss. The Mind Flayer was a group intelligence that existed in a myriad of different physical forms. These physical forms were possessed and controlled via silver-black particles, which were the Mind Flayer's means of transmission and control.

The Mind Flayer exercised supreme control over the Abyss and the Upside Down via a psychic link, with Demogorgons, vines, and Demobats forming a shared hive mind; for a period in 1985, possessed humans known as "the Flayed" also comprised part of the hive mind. This meant the entirety of the Upside Down was essentially one gigantic organism, with all its contents and agents mentally connected via the Mind Flayer.