The Mind Flayer, also known as the shadow stranger things monster, exercises supreme control over the Upside Down through a psychic link. This link connects all entities within the Upside Down, including Demogorgons, vines, Demobats, and for a time, possessed humans known as 'the Flayed'. This connection forms a shared hive mind, making the Upside Down virtually a giant superorganism. The Mind Flayer can possess other creatures by embedding fragments of itself inside their bodies, further extending its control.