Spoilers ahead for Stranger Things 3. Fans knew that the Mind Flayer wasn't gone for good after Stranger Things 2, because the final shot showed it looming in the Upside Down. And sure enough, in Stranger Things 3, the Mind Flayer is back and amassing a human/rat army. Last season, it tried inhabiting the mind and body of Will Byers, but he was eventually freed from the monster's grasp. This time, the Mind Flayer decided not to take any chances, so it possessed a ton of people. Its new hosts are called the Flayed, and they are totally terrifying.
Will, having been the only other Mind Flayer host, knows how this thing works. "He flays people," Will explained of why the monster is called the Mind Flayer "[He] takes over their mind … They basically become him." The Flayed even get super strength to help them mindlessly carry out the monster's bidding. But as Will emphasized, the Mind Flayer lies dormant until the host is needed — which is why there are times when Billy and Heather and Tom seem to be functioning semi-normally (although they still act sweaty and strange).
Billy was the first to be taken over by the Mind Flayer, and soon began bringing him more people to possess. To do so, the monster used one of its weird tentacles to basically attach itself to the human's face — leaving a small piece of itself behind. That's right, part of the physical monster lives inside the hosts. If that part is removed, the human can survive, like Will did. But most of the Flayed didn't get that chance.
Once summoned, the Flayed turn into a liquified goo of guts and bones, fusing together in small or large parts to form various sizes of the Flayer. This new embodiment made it almost impossible for Eleven to defeat the monster. She split it in two at one point, but it just glommed back together.
The biggest downfall of the Flayed (in addition to their heat intolerance) is that they're all connected. If one is injured, they all suffer the ramifications. That's actually how Nancy and Jonathan escaped two Flayed in the hospital. Jonathan was all but done for, but Nancy attacked her Flayed in an entirely different room and it also injured the Flayed attacking Jonathan. This connection is also what led Billy to die when the gate closed and killed the monster. Without a power source, host Billy couldn't survive.
The Mind Flayer actually tried to infect Eleven at one point, leaving a piece of itself in her leg, but she was able to extract it. (How dare the Mind Flayer even try.) She was in a lot of pain both before and after the attempted possession, which makes sense. When Will explained what it felt like just to sense the Mind Flayer, it sounded like the opposite of pleasant. "Everything in your body is just sinking all at once and your body goes cold and you can't breathe," he said. Yikes.
The worst part is that the Mind Flayer is still not necessarily gone. In Season 2, Eleven trapped it behind the gate — but that is seemingly an entirely different monster than in Season 3. The one creating the Flayed army is likely the leftover piece extracted from Will — or so the kids theorized. The gate was closed again at the end of the third season, but that original monster is still lurking in the Upside Down. This attempt to take over Hawkins and destroy El failed, but don't count the Mind Flayer out just yet. After all, as co-creator Ross Duffer told Entertainment Weekly following Season 2, "They shut the door on this thing but it's still out there, and it wasn't aware of Eleven and now it very much is."
The Mind Flayer tried to kill El by using the piece of it left behind in the real world. Now, perhaps the entire monster will try to find a way out of the Upside Down to finish the job. The Russians tried to open the gate in the third season — it's only a matter of time before someone else tries again. And if the Flayed army is an example of the Mind Flayer's ability to overtake the town — El and all of Hawkins better watch out.