Mind Flayer

"You shouldn't have looked for me. Because now I see you. Now we can all see you. You... let us in. And now, you are going to have to let us stay. Don't you see? All this time, we've been building it! We've been building it... for you. All that work. All that pain. All of it for you. And now it's time. Time to end it. And we're going to end you. And when you are gone, we are going to end your friends. And then we are going to end... everyone."

- The Mind Flayer while possessing Billy.

The Mind Flayer, also known as the Shadow Monster or simply "Him", is the main antagonist of the 2016 Netflix horror series Stranger Things.

It is an enormous shadow-like being that rules a supernatural dimension called "The Upside Down" and acts as the hive mind for the Demogorgon and the Demodogs.

It was portrayed by Noah Schnapp while possessing Will's body. In Season 3, it was portrayed by Dacre Montgomery when possessing Billy Hargrove.

Season 2
After Will Byers has been saved from the Upside Down at the end of Season 1, he starts having visions of the Upside Down leaking into the real world. There, inside a cataclysmic storm, he sees the Mind Flayer reaching out to him. Although Will evades the Mind Flayer several times, he eventually decides to follow the advice of Bob, the boyfriend of his mother, and face his fears. However, this leads to the Mind Flayer entering Will's body and connecting him to the hive mind. This does not only allow the Mind Flayer to spy on the real world through Will's eyes, it also causes Will to feel pain whenever one part of the hive mind is hurt, ensuring relative safety for the Mind Flayer.

The Mind Flayer spreads over Hawkins via vines that are drilling themselves through the earth in the forests near the city, creating an immense unseen vine network beneath the surface. During the season, it is temporarily able to trap Chief Hopper there, although Hopper can be saved in the last moment. It also starts developing several dog-like creatures similar to early development stages of the Demogorgon, later dubbed Demodogs, as an invasion force. After its vines are found by Chief Hopper and his allies, the Mind Flayer uses his influence over Will. He manipulates all others into believing that Will has had a hallucination of the central weak point of the vines. When the security team of the Hawkins Labs turns up there, however, it is revealed to be a trap and the team is slaughtered by the Demodogs which then round up and storm the laboratory itself. During the attack on the laboratory, Will is injected with a sleeping serum so that the Mind Flayer cannot see the enemies movements through his eyes.

In order to prevent the Demodogs from multiplying and attacking the city while also preventing the Mind Flayer vines to reach the city, Mike convinces the team that the Mind Flayer needs to be eliminated as the Mind Flayer serves as the hive mind to all the monsters. They believe that Will holds the key to defeating the beast and plan to wake him up. However, they first prepare an anonymous room so that Will - and by extent, the Mind Flayer - won't know where Will is located. After Will is woken up, the Mind Flayer attempts everything in its powers to convince the group to free him from his restraints while individual party members try to get through to Will. Despite being under the influence of the Mind Flayer, Will is able to relay the message "Close Gate". They assume that this means that the Mind Flayers influence on earth and Will is finally severed once the gate is closed. When suddenly the phone in the room rings, Will recognizes the ringtone and the Mind Flayer realizes where Will and the group are located. It immediately sends out its army of Demodogs to slay them all. Although the beasts manage to swarm the house, they are all killed by Eleven before they have the opportunity to enter.

While preparations are taken to bring Eleven into the lab in order to close the gate, the team also makes plans to make Will's body uninhabitable for the Mind Flayer so that he will leave Will before the gate is closed. The team splits into three groups. Hopper brings Eleven to the lab. Will’s Mother Joyce, Nancy, and Will’s brother Jonathan attempt to free Will. Mike, Dustin, Lucas, Steve, and Max create a distraction by burning the vines underneath the surface. Joyce and Jonathan realize that the Mind Flayer cannot stand warmth so they heat up the room, causing the Mind Flayer to scream in pain through Will's mouth. Although Will is clearly in pain, the group endures and the constant pain causes the Mind Flayer to directly act through Will. He bursts free from his shackles and attacks Joyce, starting to strangle her. To free the woman, Nancy burns Will with a heated iron rod, causing him to drop Will's mother. The Mind Flayer flees from Will's body and exits it in a gust of shadowy smoke and flees the hut.

Mike and the rest of the gang successfully reach the center of the vine system, where the Mind Flayer protects itself by having several bulbs blasting out spores as a defense mechanism. They drench the entire system inflammable liquids and burn it down. This causes the distraction they hoped for, alerting the Mind Flayer to the attack and causing it to draw off the Demodogs from the institute. This makes the way easier for the first group. Hopper and Eleven reach the center of the institute where the path is now clear. Eleven uses her powers to close the gate to the Upside Down, banishing the Mind Flayer for good. Although it once more tries to breach the gate while Eleven is sealing it, it is unsuccessful in its attempt and remains trapped in the Upside Down. With the Mind Flayer sealed away, all Demodogs on Earth drops dead immediately.

Despite having been banished back to its own dimension, the Mind Flayer is still very much alive and still seems to have intentions to invade Earth once more.

Season 3
Despite the gate being closed, some of the Mind Flayer's influence is still present in Hawkins. The creature would then be awaken due to another gate being created by a Russian machine (known as a key) under Starcourt Mall. Because of this, the Mind Flayer (as its shadow form cannot physically appear in Hawkins) proceeded to infect a large mass of rats in a mill that would later start to explode into a pool of blood and form into a grotesque biomass of red flesh to serve as its proxy body in the physical world. It would later encounter Billy Hargrove and use his body as a primary vessel to capture first a lifeguard named Heather Holloway, and then both of her parents, to be infected.

Before this, one of the rats controlled by the Mind Flayer was captured by a elderly woman named Doris Driscoll for consuming her new fertilizer, which had Nancy and Jonathan investigating on this. After the two left, the captured rat exploded into sentient flesh and it slithers from the cage. At some point, it infected Mrs. Driscoll, turning her feral and started to consume the fertilizer. Nancy and Jonathan, wanting to find the rat, discovered Driscoll in this state, having the woman be placed in a hospital. Although restrained, Mrs. Driscoll desired to “go back”, possibly referring to the mill where the Mind Flayer is.

Victims

 * Billy Hargrove (former host; killed by the Mind Flayer)
 * Heather Holloway (dissolved to flesh)
 * Doris Driscoll (dissolved to flesh)
 * Tom Holloway (dissolved to flesh)
 * Janet Holloway (dissolved to flesh)
 * Bruce Lowe (dissolved to flesh)
 * Adam (dissolved to flesh)
 * David (dissolved to flesh)

Trivia

 * Similar to the Demogorgon, the Mind Flayer is named after a Dungeons and Dragons monster, the Illithids.
 * The Mind Flayer bears a strange resemblance to the M.U.T.O.s from Godzilla. This is most likely a coincidence, however.