The Mind Flayer

The Mind Flayer, also known as the Shadow Monster, is the main antagonist of the 2016 original Netflix sci-fi TV series Stranger Things, serving as the unseen

overarching antagonist of Season 1 and the main antagonist of Season 2 and Season 3.

He is a malevolent entity that rules the parallel dimension known as the Upside Down. The Mind Flayer is a massively powerful being of unknown origin. He wields supreme control of the Upside Down, as well as the Demogorgons, tendrils, and the Flayed. With murderous intent, he repeatedly attempts to corrupt both Hawkins and the rest of the world.

The Mind Flayer first began his conquest to take over the world in 1984 when he used the Gate to spread the Upside Down's toxic biological growth underneath Hawkins, as well as possessing Will Byers and an army of Demodogs to take down Hawkins Lab. Eleven ended his plan by closing the Gate and severing the connection between the two worlds but left a part of the Mind Flayer trapped within the human world.

The part of the Mind Flayer remained dormant until it was reawakened in 1985 by the Hawkins Key. It then began flaying multiple humans and rats, feeding them chemicals, and melting their bodies into puddles of biomass that would be used to create a new body for itself to take over the world. The Mind Flayer nearly succeeded in his plan but his physical incarnation was killed when Joyce and Hopper snuck into the secret Russian laboratory and closed the new gate, severing the connection once again.

Background
The history of the Mind Flayer prior to October 1984 remains a mystery. Exactly how long he had existed for, and how and why he came to inhabit the Upside Down, is unknown. The Mind Flayer's ultimate goal and why he constantly tries to enter our world is also unclear.

1983
In November 1983, an inter-dimensional gate was opened at Hawkins Lab, allowing the Upside Down to begin to spread through. It is not known if the Mind Flayer was present within the Upside Down at this time, or if it was controlling the Demogorgon.

Will Byers, a young resident of Hawkins, was captured by a Demogorgon and brought to the Upside Down's version of the town's public library. Although he was safely recovered by friends and family, Will began to have visions of the Upside Down.

It is possible that the Mind Flayer was elsewhere during these events, and made its way toward Hawkins due to them.

1984
Will continued to have visions of the Upside Down for a year following his rescue. These visions were examined by Dr. Sam Owens at Hawkins Lab, who believed Will was experiencing post-traumatic stress disorder.

On October 29, 1984, Will and his friends went to play at the Palace Arcade. While his friends argued with the arcade's employee, Will became distracted by another vision. He walked out of the Upside Down version of the arcade. To his horror, he witnessed a blood-red storm raging in the sky above, before hearing an unearthly howl. However, Will's friend Mike snapped Will out of the vision.

Will had another vision the following evening. He went to have a bathroom break in the middle of the night and was returning to his bedroom. However, he saw a red light illuminating the walls of his house. The front door swung open, revealing the Upside Down beyond it, where the red storm violently raged once again. Will walked out into the Upside Down and witnessed the obscured but distinctive silhouette of the Mind Flayer amongst the clouds.

On Halloween night, Will experienced yet another vision while trick-or-treating in Loch Nora, a wealthy neighborhood in Hawkins. He found himself in the Upside Down version of the neighborhood. At first, Will heard a strange chittering sound coming from multiple directions. Then, he watched in horror as the Mind Flayer rose high into the air before pursuing him through Loch Nora. Will hid behind a wall, unaware that the creature was close behind him. However, Mike found Will and snapped him out of the vision before the Mind Flayer could touch him.

Will's next vision took place at school, shortly after he encountered Dart in the bathroom. Will found himself in the Upside Down once more, standing in the school corridor. Suddenly, the Mind Flayer's dark mist began to flood into the corridor before chasing after Will. After running outside and onto the field, Will stopped and turned to see the Mind Flayer looming high above the school. Remembering what Bob had told him as they drove to school, Will attempted to confront the Mind Flayer, yelling at him to "go away!" The Mind Flayer ignored Will, continuing to pursue him before entrapping him within one of its tornado-like appendages. From that point on, the Mind Flayer began to slowly take over Will's mind and body.

As time passed, Will's connection with the Mind Flayer deepened, making him frightened and distraught. Fortunately, it led Will, Joyce, Bob, and Mike to find the location of Hopper and rescuing him at the tunnels. However, due to his being a part of the hive mind, he experienced severe agony following the burning of the tendrils. It enraged the Mind Flayer as referred by an ailing Will and as retaliation, the Mind Flayer deceived Will into revealing a location that it was afraid of and exploited their connection to use him to massacre the scientists at the Hawkins Lab.

In the fallout of the Hawkins Lab massacre, the Mind Flayer escalated his conflict with the human world. Unleashing countless Demodogs into the underground tunnels, the Mind Flayer seemed hell-bent on destroying his enemies.

Meanwhile, the assembled allies retreated to the Byers house. They isolated the compromised Will in a dark room to stop the Mind Flayer from spying any further. The allies discussed the nature of the Mind Flayer and the Upside Down in a desperate bid to defeat him. As with the Demogorgon, the kids used a Dungeons & Dragons-based analogy to theorize about the Mind Flayer's nature; and thus, the nickname "Mind Flayer" was born.

The first wave of Demodogs arrived at the house, but as they did, so did Eleven. El fended off the Dogs, and after reuniting with her friends, the allies developed a three-fold plan of action against the Mind Flayer. All three of the following events unfolded more or less simultaneously, but all collectively contributed to the Mind Flayer's defeat:

The allies attempted to free Will from the Mind Flayer's influence, and thus return him to his former state. Although these attempts largely failed, Will mentally fought back against his captor and regained control of his left hand. He began to tap the side of his chair in Morse Code, which, when translated, revealed the message "CLOSE GATE". The allies realized what had to be done, sending Eleven and Hopper on a mission to close the doorway between worlds.

Joyce, Jonathan, and Nancy attempted to "exorcise" the Mind Flayer from Will's mind and body. They did this by exposing Will to extreme heat, even burning his arm with a fire poker. The possessed Will initially resisted, writhing and shrieking in pain, but eventually, the fragmented Flayer could no longer stand the heat. He fled the body and flew far, far away into the night sky.

Hopper and Eleven arrived at the now-abandoned laboratory, and as they descended through the vast depths below, the Mind Flayer's huge silhouette loomed behind the other side of the Gate. The Mind Flayer sent one of his many coils through the barrier in an attempt to stop her while simultaneously commanding Demodogs to attack them.

At the last minute, Dustin, Lucas, Max, Mike, and Steve made a plan to distract the Mind Flayer, and thus diverting his attention from the events involving Eleven and Will. They headed to the subterranean tunnels, soaking "The Hub" with gasoline and setting it aflame. This further weakened the Mind Flayer's grip on Will, as well as drawing some of the Demo-Dogs away from the Gate and Eleven.

However, Hopper managed to fend off the Dogs, while El used her powers to neutralize the coil. Finally, Eleven channeled her anger and closed the Gate, sealing the Mind Flayer's main form back inside the Upside Down. Sealing the Gate also cut off the Demodogs' connection to the Mind Flayer, killing them all almost instantly; even Dart could not survive.

With the Demodogs eradicated and the Gate sealed, the human world unknowingly enjoyed a quiet victory. With the telepathic link to his main form in the Upside Down severed, the ethereal fragment of the Mind Flayer that left Will's body became isolated and dis-empowered. This fragment form would lay dormant at Brimborn Steel Works for over seven months, until the events of July 1985. In December, during the Snow Ball at Hawkins Middle School, the Mind Flayer loomed over the version of the school in the Upside Down, seemingly spying on the Party from across dimensions.

1985
In the summer of 1985, a secret Russian operation located beneath Hawkins' newly-built Starcourt Mall successfully opened another Gate into the Upside Down. Unlike the first Gate, the new breach was too small to let through creatures and vines; however, it was still wide enough for the Mind Flayer to renew his telepathic link with the isolated piece of himself at Brimborn Steel Works. He used this fragmented form to "infect" the rats inhabiting the Steel Works and other underground areas around Hawkins, causing them to consume fertilizer, gather in swarms within the Steel Works, and then explode into blobs of biomass. The biomass fused together with the fragment form into a creature with the approximate shape and proportions of the Mind Flayer's main form. This monstrous creature served as a proxy body, through which the Mind Flayer could once again influence events in Hawkins. Later, the Mind Flayer caused Billy to crash his car, which got him to investigate the Steelworks, only to get captured and become his main host. He uses Billy to capture Heather and make her become one of the Flayed. They both later capture Heather's parents Janet and Tom. Doris Driscoll also became part of the Flayed, likely from the rat she captured. Others become part of the Flayed, including Bruce (likely from Tom, other members of the Hawkins Post may have been Flayed as well). Also, other men, women, and children became part of the Flayed at an unknown time. Will later mentions that the Mind Flayer is back, and it is eventually discovered he is controlling multiple people.

When Eleven tries to find the Mind Flayer's location through Billy, the Mind Flayer ends up talking to her through Billy's body in a Mindscape version of Hopper's Cabin. Knowing her location, the Mind Flayer melts the Flayed (except for Billy) and absorbs them to become stronger. He then goes after Eleven and her friends and attacks the cabin, managing to knock Jonathan against a wall where some of his tentacles are snapped by Eleven. However, the Mind Flayer manages to grab Eleven by her leg. As Mike, Max, Will, and Jonathan grab hold of Eleven, Nancy tries shooting at the Mind Flayer to no avail until Lucas chops off the tentacle with an ax but the Mind Flayer had managed to bite Eleven. She responds by angrily tearing the Mind Flayer's head in half, forcing him to regenerate which leaves him temporarily unable to pursue them. Eleven feels the effects of the bite but removes the piece of the Mind Flayer that was inside her leg.

Despite Eleven removing the piece of the Mind Flayer, Eleven's telekinesis is weakened and the Mind Flayer manages to find the Party after Billy tracked them due to the piece of the Mind Flayer that was in El's wound. After Billy manages to trap the Party by damaging their car, the Mind Flayer arrives at Starcourt Mall and starts hunting them down. The Mind Flayer nearly found Mike, El, and Max, but was distracted and started pursuing Nancy, Jonathan, Will, Lucas, Steve, and Robin. However, the Mind Flayer stopped his pursuit when the Flayed Billy spotted Mike, El, and Max escaping the mall through the back exit, causing the Mind Flayer to head back to the mall.

The Flayed Billy managed to trap Mike, El, and Max at a dead end before knocking all three of them unconscious and taking El hostage. Billy delivers El to the Mind Flayer who is about to kill her until the timely arrival of Nancy, Jonathan, Will, Lucas, Steve, and Robin stop him by bombarding his avatar with fireworks. As the kids fought the Mind Flayer, El managed to free Billy from the Mind Flayer's control by reminding him of a memory he had of his mother while Mike and Max regain consciousness, rushing to join the battle. When the Party runs out of ammo, the Mind Flayer tries to kill El again, but he is stopped by Billy. Enraged at Billy for resisting his control over him, the Mind Flayer proceeds to attack and kill Billy, much to the horror of Billy's stepsister, Max. After the Mind Flayer kills Billy, he suddenly loses control of his proxy body due to Joyce Byers closing the Gate underneath the mall. As the Mind Flayer loses control of his avatar, he tries to attack the Party, but fails when the Gate completely closes itself, allowing the Mind Flayer's proxy body to be killed.