The Monster, nicknamed the Demogorgon by Mike and his friends, was a predatory humanoid creature originating from the parallel dimension known as the Upside Down. When Eleven, a psychic test subject from Hawkins National Laboratory, made interdimensional contact with it, a gate between dimensions opened at the lab and the Monster entered Hawkins.
The Monster terrorized the town for a week in November 1983, abducting various residents and taking them back to the Upside Down, usually killing them. It was finally defeated and destroyed by Eleven in a showdown at Hawkins Middle School.
A test subject with psychic powers labeled Eleven was placed into a sensory deprivation tank as one of Hawkins National Laboratory's experiments; this allowed Eleven to delve into a deeper psychic state. Although she successfully eavesdropped on a Russian spy as instructed, she unwittingly came across the Monster. Eleven's instincts urged her to stay away from it; she panicked, and the experiment was aborted.
Dr. Martin Brenner was intrigued by the Monster, repeating the experiment on November 6, 1983. This time, Brenner ordered Eleven to make contact. As Eleven psychically approached the Monster, she could see it feeding off an otherworldly egg. With great hesitancy, she made contact. In that exact moment, a gate to the Monster's dimension cracked open in the wall of the tank room, allowing the Monster to pass through. Eleven managed to exit the tank and escape the lab amidst the ensuing chaos. An unnamed scientist was not so lucky; the Monster caught him as he tried to operate an elevator.
The Monster had the ability to travel between dimensions on a whim, creating temporary portals in the fabric of reality. The Monster arrived at a road near a local forest, where it stalked Will Byers on his way home. The Monster chased Will as he entered his house, forcing Will to run to his garden shed. Will hid in the shed, loading a rifle in self-defense, but the Monster simply appeared behind him, taking him into the Upside Down.
Once in the Upside Down, Will managed to evade the creature for almost a week, and began attempting to communicate with his mother. Joyce and Will had some success, with Will manipulating the electricity in his home dimension, although these attempts would sometimes attract the Monster. In an early attempt, Joyce saw her house wall become warped and distorted as the Monster started to break through, terrifying her. In a later attempt, the Monster broke clean through, although Joyce managed to outrun it.
Scientists at Hawkins Lab began investigating the dimension, discovering exactly what it contained. One such scientist entered through the Gate, wearing a hazmat suit linked to a chain rope. The scientist was savaged and killed by the Monster almost immediately after entering the dimension. The Monster hunted and killed other people and creatures, being particularly lured by blood. Barbara Holland, hunters Dale and Henry, and a deer were all victims. After Nancy Wheeler entered the Upside Down through a temporary portal, she witnessed the Monster as it fed off an egg. The Monster noticed her, chasing her until she returned through the portal.
Will eventually succumbed to exhaustion, lying in his makeshift fort. The Monster found him, breaking through the fort wall and taking him to the Upside Down library.
Nancy and Jonathan Byers later exploited the Monster's attraction to blood to ambush it. With the aid of Steve, they severely injured the beast, setting it alight - however, it survived and slipped into the Upside Down. The Monster left the Byers house and headed to the library, leaving a trail of blood.
Eleven killed a large number of government agents at Hawkins Middle School, defending herself and her friends Dustin, Lucas and Mike. The Monster was attracted by pools of spilt blood, breaking through the wall. The Monster began a spree of violence, attacking Dr. Brenner as surviving agents opened gunfire.
For a while, the four kids outran and evaded the creature, hiding in their science teacher's classroom. Eventually the Monster found them, breaking into the room and attacking them. Eleven confronted the creature, pinning it against the blackboard with her mind. After wishing farewell to her friends, she proceeded to disintegrate the beast, vanishing as well in the process.
The Monster is tall and thin with a humanoid figure. Its head has no facial features and opens like a flower to reveal nothing but teeth. It bears clawed feet and hands, and its skin appears to be coated in fluids.
After being set on fire by Nancy, Jonathan, and Steve, the Monster's body was covered in burns.
Most knowledge on the Monster is highly speculative - even Dr. Brenner admitted that he and his peers had limited understanding of the creature. That being said, the behavior of the beast is "predictable" as Brenner remarked, comparing it to a typical predator. Nancy made similar associations: she compared the creature to lions and coyotes due to their shared tendency to hunt at night. She noted that the solitary nature of the creature was similar to that of a bear.
Nancy also theorized that the Monster was attracted to blood, comparing it to a shark. This ability to detect blood appeared incredibly potent; it was aware of small quantities of it, even when separated by dimension. However, the creature was not exclusively drawn by blood; Joyce and Will Byers, as well as various Hawkins Lab scientists, were attacked by the beast despite seemingly not bleeding. It is unknown if the creature was attracted to anything else, though one distinct possibility is sound: when Nancy ventured into the Upside Down, the Monster was unaware of her presence until she made noise. Since sound from the human dimension was audible from the Upside Down, the ability to hear these noises would have greatly aided the creature while hunting. It is also possible that, like many predators, the Monster simply took any opportunity to hunt and acquire prey, and wasn't drawn by any specific stimuli.
The Monster had the ability to travel between dimensions on a whim, creating temporary portals in the fabric of reality. How or why the Monster possessed this ability is unclear, though it only became aware of the regular dimension and started creating portals after the moment of contact with Eleven. Mr. Clarke, the boys' science teacher, remarked that interdimensional travel would take "a massive amount of energy, more than humans are currently capable of creating"; going by this logic, the Monster must have wielded a great deal of energy while dimension-traveling. Nearby lights and electricity would constantly go on and off while the Monster did so, which suggests that creating these portals did indeed involve huge volumes of energy.
The Monster was hostile every time it was encountered, immediately pursuing anything which took its attention. It generally used instinct and brute strength, rather than intelligence, to get through obstacles; the main exception to this being the use of telekinesis to unlock the Byers' front door (an act which was remarkably similar to Eleven's use of her abilities). The Monster generally hunted in close proximity to Mirkwood, with the Middle School being the furthest it ever traveled in pursuit of prey. The reasons for this are unclear, though this was potentially motivated by Mirkwood's close proximity to Hawkins Lab and the Gate.
The Monster would take captured victims to the alternate version of the public library, which housed something resembling a nest. The Monster was not the only creature inhabiting this supposed nest - slug-like creatures could be seen emerging from the corpses of the Monster's victims, while Will was found with some kind of tendril extending down his throat. Although it is unclear if these creatures are related to the Monster in any shape or form, their presence in this supposed nest is suggestive of some kind of wider, symbiotic ecosystem within the Upside Down.
On numerous encounters in the Upside Down, the Monster could be seen hunkered over large, yellow eggs, feeding off them. It’s not clear whether or not these were the creature’s offspring, or if they were even the same species.
The Monster is a creature has several unique abilities:
- Strength - The Monster is a creature that has an outstanding and extraordinary strength. It can break through solid walls and is able to lunge, attack, and take down a fully grown man.
- Durability - Due to its durable skin and tough body, the Monster can withstand massive amounts of bullets from several firearms and several beatings from a baseball bat with nails. However, not even its durable skin can withstand Eleven's power of biokinesis, which seemingly killed the Monster.
- Regenerative healing - After being beaten and burning in a fire, the Monster manages to heal its injuries after a certain period of time.
- Blood detection - The Monster is strongly attracted to blood and can even sense the tiny droplets from different dimensions.
- Interdimensional travel - The Monster is able to pull individuals, such as Will Byers and Barbara Holland, into the Upside Down and can enter Hawkins through temporary tears in space time. This creates a spatial distortion similar to a wall bending outward and eventually ripping open.
- Telekinesis: The Monster occasionally used telekinesis when hunting prey. It unlocked the hatch on the Byers' front door while persuing Will, and later it pulled a wounded deer into the Upside Down using this power.
- A scientist from Hawkins National Laboratory
- A deer
- Barbara Holland
- Hunters Dale and Henry
- Dr. Martin Brenner (possibly; unknown fate)
- An uncountable number of government agents and people
Behind the scenes
The Duffer Brothers described the monster as "an interdimensional being that has more in common with the shark from Jaws than Pennywise from It. When the monster enters our dimension, it’s like a shark breaching the water. Very much like a shark, it drags its prey back into its home, where it feeds."
Many of the scenes with the monster were done practically, with Mark Steger portraying the monster in a suit with an animatronic head. CGI was used for scenes that required the monster to do things a man in a suit couldn't possibly do.
Conception and design
The Duffer Brothers had a lifelong dream to create a monster for the screen.
The artists at Aaron Sims Creative were given the task of designing the look of the Monster, with the description of a "humanoid creature with no face, just a mouth." From the very beginning, the Duffers wanted the creature to be humanoid and grotesque, with the creature designs of H.R. Giger, Clive Barker, Guillermo del Toro and Masahiro Ito being influential to the Monster’s appearance because of their "strangeness."The artists went through several early concepts, some of which were "more monstrous in shape, even more asymmetrical." Ultimately, they were inspired by nature when they came up with the flower petal-like mouth in the first week, a design which really resonated with the Duffers. 
- See Also: Special Effects
Having grown up on genre films before computer graphics, the Duffer Brothers wanted to go back to using practical horror elements and using a person in a suit that could interact with the actors in real time. "There was something about the effects being so tangible in those films that made them especially terrifying to us when we were kids."
Spectral Motion, a makeup and creature effects studio which has worked on productions such as Hellboy and Pan’s Labyrinth, was given the task of building the Monster. They only had a span of two months to complete the project, leaving almost no room for error.
The process began with a scan of Mark Stegar's body, from which a full-size form was created that the body of the Monster was sculpted onto. Animatronics were installed in the head and arms, which had to be operated with a remote control. Stegar often had to wear metal stilts that elevated him nearly ten inches. Despite the suit's complexity, it only took half an hour to 40 minutes to put on and only weighed approximately 30 pounds.
The animatronic head consisted of 26 motors, and the noise they produced often made hearing directions difficult for Stegar. The animatronics were also designed in such a way that the “petals” would move uniquely each time they opened. When the Duffers first saw the Monster's mouth open, they were blown away, saying “It felt organic. Creepy. Real.” For the shots where the creature's mouth was open, Stegar's exposed face would be removed digitally and replaced with a mouthpiece that was also created by Spectral Motion.
The CGI enhancements and effects were handled by Aaron Sims Creative.
- The Monster received its nickname from Eleven using the Demogorgon game piece to show that Will was hiding from it.
- It is currently unknown how exactly the Monster can sense blood, as it appears to only have a "mouth" and no other visible orifices that could be used for scent.
- Though the Monster can use telekinesis, it only exercises this ability twice: when it unlocked the Byers' front door and presumably when it pulled in the deer.
- While the Monster exclusively hunted at night, Nancy saw it at daytime as she looked for Barb. This was done for storytelling purposes.
- The Monster was killed in Mr. Clarke's classroom.
- It was hinted that the falling ash-like spores present in the Upside Down have a possible direct connection to the Monster. 
- The Monster's open mouth somewhat resembles a Rafflesia arnoldi, a flower with the odor of decaying flesh, earning it the nickname "corpse flower." Furthermore, its teeth are similar to those of the leatherback sea turtle.
- In the original pilot script, people would bleed from their nose and ears when in contact with the Monster, similar to how Eleven bleeds when using her telekinetic powers.
- In the Dungeons & Dragons game in Chapter One, Will uses fireball against the Demogorgon, possibly foreshadowing Nancy, Jonathan, and Steve setting the Monster alight in Chapter Eight.
- The twin toddlers who portray Holly Wheeler, as well as Millie's younger sister Ava, were initially frightened by the practical Monster suit. To calm them down, a crew member told them that it was a benevolent monster from the Disney/Pixar film Monsters, Inc.
- If one looks closely, it can be seen that the shot of the Monster attacking Barb in "Chapter 2" was reused in "Chapter 6", when the Monster notices Eleven's presence. This was possibly done because of budget limitations or time constraints.
- When asked if the Monster was some kind of plant due to its appearance, Mark Steger responded that it was a fair speculation, adding that he feels it's "more mushroom, which is kind of between plant and animal. "
- ↑ The Duffer Brothers said in an interview that "Obviously something happened to [Eleven] when she destroyed and killed that monster," effectively confirming its status.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 "Stranger Things episode 6: How the Duffer Brothers created their monster" Entertainment Weekly. July 20, 2016
- ↑ "Netflix's Stranger Things: Shawn Levy interview" Den of Geek!. July 15, 2016.
- ↑ 4.0 4.1 4.2 "Exclusive ‘Stranger Things’ Art Reveals ‘Upside Down’ Secrets and Barb’s Alternate End" Screencrush. August 22, 2016.
- ↑ 5.0 5.1 "‘Stranger Things’ Finale: Duffer Brothers Talk Cliffhangers, Death and Season 2" Variety. July 18, 2016.
- ↑ "Meet the man behind Stranger Things' terrifying monster" The Verge. August 14, 2016.
- ↑ 7.0 7.1 7.2 "How 'Stranger Things' Built Its Terrifying Monster" Vice. August 9, 2016.
- ↑ "‘Stranger Things’ Demogorgon: Meet the Man Behind the Upside-Down’s Faceless Monster" IndieWire. August 27, 2016.
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