Though fans suspected as much, an official Stranger Things behind-the-scenes companion guide called Worlds Turned Upside Down has confirmed that the Mind Flayer, aka the Shadow Monster, will indeed be returning for Season 3.
Spoiler warning: spoilers for Stranger Things Season 2 ahead.
As reported by Screen Rant, the book’s epilogue makes some pretty concrete teases for the upcoming season. “The Mind Flayer hasn’t lost interest in Hawkins,” it reads. “The tentacled creature continues to loom near Hawkins inside the Upside Down, and its malevolent influence isn’t absent from the new season.”
Source: IGN.com Stranger Things Season 3 Will Bring Back Mind Flayer