The Hawkins crew might have thought the Demogorgon was the worst foe they could ever face during the first season of Stranger Things. But then Season 2 leveled up with the monstrous Mind Flayer. But what is the Mind Flayer, exactly? Stranger Things never quite offered a satisfactory answer in Season 2, but Season 3 might delve deeper into the monster.
The Mind Flayer (a.k.a. Shadow Monster) was a massive, multi-tentacled creature who lived in the Upside Down, though its reach extended far beyond. It had psychic abilities that allowed it to control an army of Demodogs, creating a hive mind that could carry out its whims. It was also able to possess Will Byers and use him as a way of keeping an eye on everything happening in the, shall we say, Rightside Up. The Mind Flayer appears to be in control of the Upside Down, making it a big threat that needs to be defeated.
But not much is known about the Mind Flayer. So far, most of the effort has gone into stopping the monsters from the Upside Down, not figuring out their grand plans for taking over small Indiana towns. But Season 3 might have more to say about the monster's motives.
At the end of Season 2, the Hawkins kids thought that they had banished the Mind Flayer for good. Thanks to Eleven's efforts, it was sealed into the Upside Down. She managed to close the rift that it was leaking out of, thus keeping it within its alternate dimension prison. Or so it seemed. One of the final shots of the season showed the Mind Flayer looming over the Hawkins school in the Upside Down, indicating that its attempts to break through weren't over.
According to Screen Rant, the official Stranger Things companion book, Worlds Turned Upside Down, confirmed that the Mind Flayer would be back for Season 3. It wasn't defeated at all; a small battle may have been won, but the war waged on. "We ended Season 2 with a clear signal that the Shadow Monster was not eliminated, and maybe he's even identified his foe," said producer Shawn Levy. "And that darkness, and the battle that it will require, grows in Season 3."
In fact, one of the trailers for Season 3 appeared to be narrated by the Mind Flayer itself. Will feared that they hadn't actually locked the creature in the Upside Down, but instead trapped it in their world. Now it was looking for a new host to possess like it did Will. In a surprisingly calm and poised voice (does the Mind Flayer do podcasts? I'd listen.), the monster explained that it was out to kill everyone, essentially. It wanted total destruction. And Eleven seemed to believe it was "building something" to do just that, though the specifics weren't revealed.
The Mind Flayer is after what every super villain is: power, influence, control, and a lot of bloodshed. It may have more tentacles than your average bad guy, but its goals are the same. What it is exactly — ancient god, giant spider, clown demon — remains to be seen.