Warning: Stranger Things season 3 spoilers are ahead.
The kids in Hawkins have battled Demogorgons and the Mind Flayer, and Stranger Things season 3 introduces us to a brand new monster. And its name is... the monster, though technically it's tied to the Mind Flayer.
Breaking from Stranger Things tradition, the new monster for this season isn’t given some sort of scary, ominous name and is instead only referred to as “the monster” all season long. It does come with (and from) some Mind Flayer-possessed minions though, who thanks to their relation to the Mind Flayer, are referred to as the Flayed. The monster, built of its flayed counterparts, might be the most dangerous demon the kids have faced yet.
All three of the introduced monsters so far have had their own set of abilities and powers, and the Season 3 monster is maybe the most complicated to pin down. It’s ever changing, ever moving, super gooey, and constantly growing in size making it more and more powerful with each passing day. While we know there are multiple demorogrons (and demo-dogs), and one giant Mind Flayer, anyone — human or animal — can become a part of this brand new monster. After a tainted batch of fertilizer gets into Hawkins, rats start eating it and going rabid. Then people start eating it, too. And then the Monster marks Billy (Dacre Montgomery) as the new epicentre host and he starts turning more and more people into this possessed, Flayed state until they all join together to create one giant, oozing monster. How fun for everyone involved!
Warning — this is where the spoilers get really intense.
What does this monster want?
In the words of the monster — via the Flayed version of Billy — ”to build.” The monster’s not looking to build anything concrete, like say another Star court Mall. Rather, the monster wants to build itself. It tasks Billy with finding new recruits in Hawkins but not for say, an army. Rather, the monster is looking to increase its size and scope and it does so by absorbing those who have been turned. (Those who have been turned are referred to as “The Flayed” after the Mind Flayer, as they’re not in total control of their movements and actions.)
The monster started out with just the rabid rats in the town but after Billy and Heather (Francesca Reale) bring a lot of other residents to it, one by one they turn to ooze and join in with the monster host. We see this happen in Episode 6, “E Pluribus Unum,” which translates to “Out of many, one.” It’s exactly what the monster is doing.
Does the monster have powers?
Aside from killing and ripping things apart, the monster doesn’t have any sort of crazy powers aside maybe moving very fast... and you know, assembling itself from rats and possessed people. Its most terrifying trait is possible that it is growing in size with each new person that joins it, and parts of it can manifest anywhere (like when Nancy and Jonathan are being chased through the hospital).
Is the Mind Flayer controlling the monster?
Yes, the Mind Flayer was essentially controlling the monster, since the Mind Flayer appears to be THE big bad of the Upside Down. As the kids deduce, even though Eleven closed the gate and the Mind Flayer vacated Will’s body, part of it is still out there in the world. And even though the monster is dead, the Mind Flayer is still alive.
Is the monster dead?
The monster sure looks dead at the end of the season. After Joyce destroyed the machine trying to open the gate for the Upside Down, the monster falls to the ground — so their plan worked. Now how the monster was removed from the destroyed Starcourt Mall is another story and unfortunately, we don’t see that. There’s a high chance that the U.S. Government is running experiments on the monster’s body, and let’s hope nothing goes wrong there.
How many people did the monster kill?
That’s unclear, but it’s a lot of people. According to a news report, 30 people died in a “fire” at the Starcourt Mall, but it’s unclear if that refers to any of the Russians in the underground base or actual people who had flayed and joined with the monster.