Dart died when the gate was closed, along with everything else that came through from the Upside Down. The Mind Flayer (a.k.a the Shadow Monster) is basically a hive queen. The entire biosphere of its world - all the animals and plants - are connected by an "infection". Maybe it's a virus, or something fungal, whatever it is it serves the purpose of linking them to the hive mind. Imagine if every tree, every bug, every blade of grass, every critter in the woods, basically all life on Earth, was a single massive hive with a collective consciousness. Like the Borg on Star Trek. If a little birdy has eyes on you, every bear in the forest knows where you are. We've only seen the vines and the demogorgons (its soldiers) so far, plus the Shadow itself. I think we can assume there's a wide variety of other creatures that are part of the hive. What the Mind Flayer did to Will was transfer part of its own mass into him, the same infectious agent that links its other creatures together, assimilating Will into its hive.

In the Upside Down those fast growing vines are everywhere, on just about every surface. And down in the tunnels we saw that they make those spores that fill the air. So when a new world is overrun the vines spread and spores are released into the atmosphere. The perpetual cloud of them gets thicker and thicker. Most of the sunlight gets blocked out and it turns dark and cold, basically a permanent nuclear winter. The Mind Flayer's new conquest is now terraformed and the native life can't survive there anymore. This is what happened in the other world, when it invaded. And the same thing would've happened here if El hadn't slammed the door shut.

We haven't seen the last of the Mind Flayer. It's a world killer, and somehow I doubt it ever gives up. When it finds a way back its top priority is going to be taking out Eleven - this may be the first time it's ever been booted off a world it already started occupying, no way is it going to let that happen again!

@chrisjdel said:

Dart died when the gate was closed, along with everything else that came through from the Upside Down. The Mind Flayer (a.k.a the Shadow Monster) is basically a hive queen. The entire biosphere of its world - all the animals and plants - are connected by an "infection". Maybe it's a virus, or something fungal, whatever it is it serves the purpose of linking them to the hive mind. Imagine if every tree, every bug, every blade of grass, every critter in the woods, basically all life on Earth, was a single massive hive with a collective consciousness. Like the Borg on Star Trek. If a little birdy has eyes on you, every bear in the forest knows where you are. We've only seen the vines and the demogorgons (its soldiers) so far, plus the Shadow itself. I think we can assume there's a wide variety of other creatures that are part of the hive. What the Mind Flayer did to Will was transfer part of its own mass into him, the same infectious agent that links its other creatures together, assimilating Will into its hive.

In the Upside Down those fast growing vines are everywhere, on just about every surface. And down in the tunnels we saw that they make those spores that fill the air. So when a new world is overrun the vines spread and spores are released into the atmosphere. The perpetual cloud of them gets thicker and thicker. Most of the sunlight gets blocked out and it turns dark and cold, basically a permanent nuclear winter. The Mind Flayer's new conquest is now terraformed and the native life can't survive there anymore. This is what happened in the other world, when it invaded. And the same thing would've happened here if El hadn't slammed the door shut.

We haven't seen the last of the Mind Flayer. It's a world killer, and somehow I doubt it ever gives up. When it finds a way back its top priority is going to be taking out Eleven - this may be the first time it's ever been booted off a world it already started occupying, no way is it going to let that happen again!

I wonder if it will target her loved ones, like her possible new family (if Hopper and Joyce do get together next season) and friends just to cause her some emotional pain

@Theother said:

I wonder if it will target her loved ones, like her possible new family (if Hopper and Joyce do get together next season) and friends just to cause her some emotional pain

The Mind Flayer is a very H.P. Lovecraft kind of monster. Ancient, mysterious, and totally alien. It's thoughts and motivations are probably incomprehensible to us (and vice versa). We do know it experiences anger at being hurt, and probably frustration at being thwarted, and that it regards humans as vermin to be exterminated. I think it will either try to chip away at Eleven's defenses and take control or failing that, kill her so she can't interfere again.

Although they could go another way with it. The surprising discovery that some humans are actually formidable might spark an interest in understanding us - so it can conquer us, of course, but the Mind Flayer might start appearing to Eleven. In the form of Brenner perhaps. Talking to her, probing for her weaknesses. That sort of thing. In the process we would learn things not only about El but about the hive and its history, which could be very interesting stuff.

It could infect more people and lure them into the pumpkin patch.

Also I think the upside down world will come back into play since Mike & Eleven use it to hear each other. It could use that to travel without needing a host and find away pass the barrier.

The mind flyer is going to want revenge for them interfering in it's plans. Will was double teaming him and Eleven closed the gate. It probably wasn't used to resistances and may lead to them going into hiding for a few eps.

I think the vines growing underneath were secreting toxins into the soil, then the pumpkin plants absorbed the stuff through their roots and died. Something alkaline or acidic maybe.

That place Eleven goes to, where it's dark and there's water on the ground, that's not the Upside Down. Think of it as her mental desktop. It's all in her head. When she focuses on a person, she sees them there, sometimes a few objects in their vicinity and that's it. Not other people they're interacting with or things happening close by. When she's not focused on anyone else she's there all by herself - because it's not a real physical place.

The Mind Flayer is probably feeling some combination of pissed off and freaked out by what happened. This is a totally unanticipated complication. Not to mention a threat of unknown magnitude, possibly putting its very existence in jeopardy. It will have to deal with Eleven before it can invade successfully. Exactly how it does that, we'll have to wait and see.