Will's Drawing Of The Mind Flayer

"	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 are 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 through Billy to Eleven

The Mind Flayer, also known as the Shadow Monster ^[1], is one of the two main antagonists (alongside Martin Brenner) 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 the second and third seasons.

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 and removing The Mind Flayer from Will's body.

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.

1 History
- 1.1 Background
- 1.2 1983
- 1.3 1984
- 1.4 1985
2 Appearance
3 Incarnations
- 3.1 The Isolated Particulate
- 3.2 The Spider Monster
- 3.3 The Hospital Monster
4 Nature
5 Powers
6 Relationships
7 Appearances
- 7.1 Comics
8 Behind the scenes
- 8.1 Concept and design
- 8.2 Name
9 Memorable Quotes
10 Trivia
11 Gallery
12 References

History

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 corrupted 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.

A drawing of the Mind Flayer, drawn by Will Byers.

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

Main Article: The Spider Monster

The fragment of the Mind Flayer awakening.

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 that detached from Will at the end of Season 2 and landed in 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.

The Mind Flayer's proxy body lying dead due to the Gate being closed once again.

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.

Appearance

The Mind Flayer is a gargantuan entity, being at least 50 stories tall. His body appears to be entirely composed of minuscule particles, giving him a misty appearance akin to a shadow. It is unknown if the Mind Flayer is entirely made of smoke, or if there is any physical anatomy to the creature. The creature has multiple limbs, some of which branch into several smaller appendages; these limbs visually resemble the tendrils and vines present in the Upside Down. He seems to have an elongated, flame-shaped head that sprouts from the central body that bears no visible facial features. His presence is often accompanied by unnatural red lightning, which Will describes as a "storm".

Incarnations

Through his telepathic link, the Mind Flayer could directly control additional entities and beings:

The Isolated Particulate

Pockets of the silver-black mist could move independently while remaining under the control of the Mind Flayer. One of these pockets left the main body and latched itself onto Will Byers, allowing the Mind Flayer to possess him. Will's friends and family eventually forced the creature to abandon Will, with the mist fragment flying away into the sky. The mist fragment eventually re-awakened at Brimborn Steel Works when the Starcourt Key was activated in the Summer of 1985, renewing the mist fragment's telepathic link to the main body. The Mind Flayer, via the mist creature, immediately summoned hordes of rats to the Steel Works, melting down their bodies and creating the Proxy Body.

The Spider Monster

The Spider Monster, the Mind Flayer's 1985 proxy body, resembled a gargantuan, monstrous spider. This incarnation of the Mind Flayer closely matches its Upside Down counterpart in terms of its shape and proportions. However, unlike the Flayer's main body, the Spider Monster is not shrouded in silver-black mist, nor is it as large (though it still stands tall at over 20 feet in its fully-formed state). The Spider Monster is nigh on indestructible but remains vulnerable to heat.

The Spider Monster is composed of the bodies of the Flayed: dozens of congealed organisms, both human and animal, who were converted to serve the Mind Flayer's will. The Spider Monster was originally relatively small, being composed purely out of Flayed rodent bodies; however, he rapidly grew into his fully-fledged state after finally consuming the converted humans.

The Hospital Monster

Essentially, the Hospital Monster was a smaller version of the Spider Monster, composed of the Flayed humans Tom Holloway and Bruce Lowe. Unlike the Spider Monster, the Hospital Monster did not resemble the Mind Flayer but rather had a more asymmetrical appearance. The creature was ultimately absorbed into the Spider Monster.

Nature

The Mind Flayer is a sentient entity that possesses intelligence equal or superior to that of a human. This sets him apart from the Demogorgons, which appear to behave purely animalistic when he is not exercising his control over them. The creature was highly hostile toward anyone and anything that threatened his plans; using his almost omnipotent intelligence and dominion over the Upside Down to his advantage, he would develop ways to thwart and outsmart his adversaries. Such as, using his possession over Will as a way to spy on Hawkins Lab and luring a team of soldiers into a trap. The Mind Flayer has also been shown to be highly vindictive, such as when he centered his entire plan on personally destroying the Party, and directly threatening Eleven through Billy inside her own psyche.

While much of the Mind Flayer is currently shrouded in mystery, through Will's connection to the entity, it is known that the Mind Flayer wishes to eradicate humanity and merge Earth with the Upside Down. While its true motivations for doing so are yet to be determined, through comparing the entity to the synonymously named creature from Dungeons & Dragons, Dustin theorized that the Mind Flayer viewed himself as superior to all other life. Exactly how accurate this theory remains to be seen.

While a massively powerful being, the Mind Flayer was not without his weaknesses. Most notably, he has a low tolerance to heat, particularly fire. All hosts within his hive mind share this weakness, resulting in him feeling pain whenever a host is exposed to heat or burned.

Fundamentally, The Mind Flayer seems to be nothing but a pure evil monstrosity that seeks nothing but total domination and does not value any form of sentient life other than himself.

Powers

Hive Mind: The Mind Flayer's most distinctive feature is the ability to take control and command of the creatures from the Upside Down and later Will. Upon having its feeding area burned, he sent multiple Demodogs to the tunnels in order to deal with the problem. He also had them try and stop Eleven from closing the Gate. The downside of this mental connection was that the burning of tendrils would affect the Demodogs and Will, and most likely the Mind Flayer itself. Will was also able to learn instinctive knowledge of its goals, the tunnels, and its weakness. Despite the hive mind's reach, when Eleven sealed the rift between worlds, the connection was severed and led to the death of its subjugates.

Possession: By imbuing a portion of its essence on an affected subject (despite both the Mind Flayer and Will being in separate worlds), the Mind Flayer is able to link the infected to the rest of its army. It appears to do this by forcing it's essence into it's desired host and slowly spreading throughout their body over a period of time. An example of his power was how he tricked Will into sending numerous soldiers to their death by having Will tell them its weakness was within the feeding area in the tunnels. The possession can be severed by subjecting the infected to extreme heat, which burns the essence out of its body by making it uninhabitable due to a weakness to heat and light. However, when a host is under threat of being discovered or severed from the Mind Flayer's possession, its influence can force the host to fight back; it gave Will inhuman levels of strength, allowing him to remove its restraints and throttle Joyce effortlessly with one hand while she attempted to free Will from the creature's possession. When the Mind Flayer later possessed Billy Hargrove, the Mind Flayer gave Billy enough strength to bust down the door of the sauna room he was trapped in, even temporarily resisting Eleven's telekinesis and throwing a barbell at her. The affected subjects also seem to take damage that would otherwise kill a human, such as eating chemicals. Billy, under the Mind Flayer's control, was strong enough to survive getting thrown through a brick wall at a high speed, and even survive getting hit from the side by a car. One further aspect of possession is that the Mind Flayer learns from the memories of those it possesses, as proven by its controlling of Tom Holloway ,once possessed, to lie about the family of Doris Driscoll "threatening litigation" to Nancy and Jonathan, a very specific human terminology that the Mind Flayer would not have learned anywhere in the Upside Down. This is particularly concerning if the Mind Flayer is able to return to the world once again having learned about human vehicles and technical weaponry; despite their abilities all of its "troops" so far have been, strictly speaking, hand-to-hand fighters that travel on foot, which are two extreme tactical disadvantages against modern weaponry and vehicles. In some occasions, as shown with Heather Holloway, the host may experience changes in its appearance such as larger muscles, prominent neck bones and larger heads and shoulders. A host's eyes can also become very dilated and the iris will become surrounded by dark blue veins.
Biological Change: Hosts of The Mind Flayer are shown to go through abnormal and otherwise fatal adjustments to their body. Season 2 reveals that Will's brain had been invaded by an unknown anomaly soon after he was flayed, and it can be assumed that the rest of the flayed suffered the same consequence. This potentially allows The Mind Flayer to control what its host thinks, feels, and senses. Human Hosts are also shown to have their circulatory system compromised by the essence of The Mind Flayer, allowing easy spread and control of the host, as well as giving their veins and skin a dark appearance.

Flayed Impairment: Under the influence of the Mind Flayer, Billy's body is shown to be unable to repair injuries that it has sustained. This is shown after the Sauna incident, where Billy's arm and face are abraised; these injuries remain throughout the rest of his role in the show. It is unknown if this applies to any other aspects of the body, or if this is the case with all of the Mind Flayer's subjects, as it could explain why the Demogorgon/Demodogs have a sickly appearance.

Biokinesis: Through sheer force of will it can melt host bodies and maintain a proxy body, and even create smaller monsters under its control that can later be re-absorbed into the main proxy body. This is theorized to only be possibly after the consumption of fatal chemicals and/or dioxide.

Weather Manipulation: When Will Byers witnesses the Mind Flayer in the Upside Down, the creature is often surrounded by a large storm consisting of red lighting and thick black clouds. While the nature of these storms is not entirely clear, it is possible the Mind Flayer is responsible for their creation. The storm is possibly based on the Mind Flayer's emotions, the Mind Flayer was seen looming over Hawkins Middle School, seething with anger.

Power Negation: After biting her with one of its tentacles, the Mind Flayer's proxy left a piece of itself inside Eleven, severely weakening her telekinesis later that evening. Even three months after defeating the creature, Eleven's powers were not back to full strength. How the Mind Flayer's proxy gained this ability is unclear, but it is possible that the chemicals the Flayed ingested played a part.
Bio Technology Manipulation: The Mind Flayer has proven very, very adept at genetic analysis and manipulation. When it was effectively defeated by Eleven at the first Gate, within the span of roughly one year it fully developed the concept for the proxy body, a brand-new being created from melted animals, humans and chemicals, specifically designed to combat Eleven. The entire Demogorgon species was likely either conquered and modified by the Mind Flayer long ago or its direct custom creation, with both possibilities involving extensive genetic manipulation and design. The extensive vine networks are likely the same.

Relationships

Ever since Eleven closed the gate in front of it in 1984, the Mind Flayer held a deep-seated grudge against her, viewing her as a threat to its plans and creating an army for the sole purpose of destroying her.

When Will was trapped in the Upside Down and captured by a Demogorgon, the Mind Flayer became interested in Will and decided to use him for his own purposes, showing no care for him. After Will's rescue, the Mind Flayer started spreading the Upside Down through Hawkins where Will was given visions of the Upside Down. The Mind Flayer eventually confronted Will and started to seek him as a vessel in order to use him to pave into the real world. The Mind Flayer was thwarted two times by Will's friend, Mike, but when Will finally decided to confront the Mind Flayer, he became possessed by a piece of the Mind Flayer. The Mind Flayer slowly took over Will's body and mind, allowing to see and feel everything the Mind Flayer was doing. After Will took advantage of his possession to rescue Jim Hopper where the tunnels were burned by the government soldiers, the Mind Flayer used his possession of Will, forcing him to trick the soldiers into going a trap, as revenge for hurting him through the burning of the tunnels. The Mind Flayer tried to keep his control over Will intact but was stopped by Will's family and Nancy Wheeler as they burned the Mind Flayer out of Will.

A year later, in 1985, despite being freed from the Mind Flayer, he left a huge impact on Will as he could still sense the Mind Flayer's presence whenever he was close. Nevertheless, The Mind Flayer did not seem to remember Will, or harbor any resentment toward him. However, the piece of the Mind Flayer that possessed Will constructed a body for himself out of rats and the Flayed so he could kill Will and his friends as revenge for the events in 1984. However, his plans were thwarted when Joyce Byers closed the Gate, killing the Mind Flayer's proxy body.

Upon its return to Earth in 1985, the Mind Flayer lured Billy into its new lair, converting him into the first Flayed and using him to turn other residents of Hawkins, as well. Unlike the rest of the Flayed whom the Mind Flayer had killed to build his proxy body, he kept Billy alive possibly due to him being his main human host, as well as his fortunate physical buff and being the stepbrother of Max Mayfield, a member of the Party. The Mind Flayer did not seem to care for Billy as a person and quickly killed him after Billy broke free from its control through the memory of his mother.

Appearances

Season Two appearances
MADMAX	Trick or Treat, Freak	The Pollywog
Will the Wise	~~Dig Dug~~	The Spy*
~~The Lost Sister~~	~~The Mind Flayer~~	The Gate

*The Mind Flayer appears during a flashback in The Spy.

Season Three appearances
Suzie, Do You Copy?	The Mall Rats	The Case of the Missing Lifeguard	The Sauna Test
The Flayed	E Pluribus Unum	The Bite	The Battle of Starcourt

Comics

Volume 1 appearances
~~Issue 1~~	~~Issue 2~~	~~Issue 3~~	Issue 4

Behind the scenes

Concept and design

Matt Duffer told Entertainment Weekly that the shadow monster was inspired by horror classics: "There's an H.P. Lovecraft sort of approach, this inter-dimensional being that is sort of beyond human comprehension."^[2]

Concept art of Will confronting the Mind Flayer on the field.

Visual-effects producers Paul and Christina Graff were tasked with designing the creature. Because the first season incorporated several electrical interferences, they knew the second season would involve storms. Once it was decided that the monster was going to be "storm-like", the Graffs began looking to references of storms and tornadoes. Steve Messing, a matte painter who was also consulted on the designs of several other Stranger Things elements, created concept art of the creature based on tornadoes, volcanoes, and other natural forces. In addition, they were inspired by volcanic eruptions with lightning storms in South America.^[3]

Name

While the entity had been referred to as the Shadow Monster throughout production, the Duffers eventually decided to give him a proper name. Looking through the Dungeons & Dragons manual, they decided to name the creature after the Mind Flayer due to how similar it was to their concept for the Shadow Monster.^[4]

Memorable Quotes

"He can't hear you. 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."

"To build. I want you to build. What you see."

Trivia

It's possible that to be infected, one needs to have a tendril similar to the Library Tendril attached to them first, or have been in the Upside Down for a significant amount of time; the exact nature of the infection has not yet been confirmed.
Possessed humans are colder to the touch than average, to the point where the temperature around possessed humans significantly decreases.
Heat and light are some of its weaknesses. Extreme heat causes the human extreme pain but is able to cure them with enough exposure and heat. Bob Newby also noted that the Mind Flayer's vines beneath Hawkins deliberately avoided contact with bodies of water, meaning it is also likely hydrophobic with an allergy/aversion to water.
Dart, Will, and Billy showed the ability to fight or ignore the Mind Flayer's influence. Dart was able to remember Dustin, Will was able to communicate in Morse Code while being reached emotionally by his family and Billy was able to fight the Mind Flayer, even though he died. A common feature among these moments is the infected being reminded of positive or happy memories(Dart remembering Dustin feeding him nougat, Will being reminded of happy memories involving his friends and family, Billy being reminded of his mother) these moments, along with how the Mind Flayer storm in Billy's mind was filled with traumatic memories from Billy's childhood, seem to suggest that the Mind Flayer feeds on negative emotions and memories.
In Dungeons and Dragons, Mind Flayers are a race of psychic humanoids with tentacles on their faces who eat brains. They primarily use their powerful psychic abilities to mentally enslave other beings. More recent editions have given the Mind Flayers the proper name "Illithids".
The scene in which the Shadow Monster infects or enters Will bears resemblance to similar possession sequences in horror films, most notably in 1973's The Exorcist.
A possessed Billy uses "we" pronouns to refer to itself, suggesting that the Flayed may maintain vestigial traits of individuality even while part of the Mind Flayer's hive mind.
The Mind Flayer seems to have such vast psychic power that it is able to detect Eleven spying on him through the void even though Eleven didn't make any contact or make the Mind Flayer aware of her presence. He is also able to hide himself and his hosts from Eleven's spying, and can even trap Eleven inside the void.
The Mind Flayer's abilities at genetic control and rapid manipulation, combined with its extensive psychic abilities, are very, very similar to the Zerg Overmind from the Starcraft videogame series, and it is very likely just as dangerous.
Despite having an episode named after it, the creature itself is not present in said episode
The Mind flayer is very similar to the Elder gods of H.P. Lovecraft, with his work actually tying into the stranger things universe, with lisa [1]from the novel "darkness at the edge of town", being a graduate of the infamous lovecraftian college "Miskatonic university"

Gallery

References

↑ "Stranger Things 2: Behind the sequel's big, bad 'shadow monster'" Entertainment Weekly. September 28, 2017.
↑ "Stranger Things 2: Behind the sequel's big, bad 'shadow monster'" Entertainment Weekly. September 28, 2017.
↑ "How Stranger Things 2 Transformed Hawkins Into the Upside Down" Vulture. November 8, 2017.
↑ "The Duffer Brothers Recap Stranger Things 2, 'Chapter Eight: The Mind Flayer'" Vulture. November 8, 2017

The Upside Down
Locations	The Upside Down • The Gate • Hawkins • Hawkins National Laboratory • Public Library • Portals • Hawkins Middle School • Hawkins Tunnel System
Entities	D'Artagnan • The Demogorgon • Demogorgon (species) • The Flayed • Hospital Monster • The Mind Flayer • Russian Demogorgon • The Spider Monster • Hawkins Tunnel System • Vines
The Flayed	Billy Hargrove • Heather Holloway • Bruce Lowe • Tom Holloway • Janet Holloway • Doris Driscoll
Visitors	Joyce Byers • Will Byers • Eleven • Steve Harrington • Dustin Henderson • Barbara Holland • Jim Hopper • Max Mayfield • Bob Newby • Shepard • Lucas Sinclair • Mike Wheeler • Nancy Wheeler
Miscellaneous	Upside Down Egg • Dungeons & Dragons • The Void • The Keys
Articles may contain spoilers.