Every Monster on "Stranger Things" Explained

The big bad of seasons two and three is a tentacled monstrosity that not only attacks physically but can also possess and control his victims. A huge part of season two is devoted to his growing control over Will and the gang's attempts to free Will from his control. In the third season, he's back and controlling more residents of Hawkins, including Max's stepbrother, Billy, before finally being defeated by the combined efforts of all the Hawkins heroes. The Mind Flayer is, essentially, the big "boss" of the monsters from the Upside Down throughout the first three seasons, controlling them in apparent hopes of wiping out all of humanity.