Remember the Demogorgon? Well, that asshole was just a taste of what the Upside Down had in store for our heroes. Stranger Things 2introduced two significant new creatures: demodogs, four-legged Demogorgons; and The Mind Flayer, otherwise known as the Shadow Monster, a giant arachnid-like monster who appears to be the powerful final boss in the Upside Down. It's kinda like the brain down there. The Mind Flayer, named after a Dungeons & Dragons creature with psionic powers, first appeared to Will in visions, or "now-memories," and it later possessed him, linking them and gradually turning little Will Byers into a serious threat over the course of the season.
Even though the monster is eradicated from Will's body by the end of the season, thereby allowing Eleven to close the main gate to the Upside Down, it's not gone for good. In the final moments of the season, The Mind Flyer is seen lurking over Hawkins Middle School in the Upside Down, no doubt plotting the destruction of Hawkins and its protector, Eleven.
While it's unclear if The Mind Flayer is the show's ultimate Big Bad, it's almost certainly going to rear its ugly head again in the third season. The Duffer Brothers have been candid about their four-season plan, so unless there's another monster hiding in the Upside Down, it's safe to assume that The Mind Flayer is the key to all that nonsense. But maybe — just maybe! — there are multiple Mind Flayers.
Steve, you're gonna need a bigger bat.