The Monster, nicknamed the Demogorgon by Mike and his friends, was a creature originating from the parallel dimension known as the Upside Down. When Eleven, a psychic test subject from Hawkins National Laboratory, made inter-dimensional 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 a strange 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's body into a thick mist - a mist in which she too vanished.
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.
Much like its home dimension, what is known about the Monster is mostly speculation. Even Brenner admitted to not fully understanding the creature, but also said its behavior is predictable, comparing it to a typical animal. Likewise, Nancy compared the way it hunted at night to a lion and coyote, and its solitary nature to that of a bear.
Nancy also theorized that the Monster was attracted to blood like a shark. While this did appear to be true, it didn't seem to be the only thing that lured the Monster, as neither the scientist nor Will were bleeding when they were taken. Later, the Monster repeatedly pursued Joyce even though there was no indication she had been injured. It is unknown what else attracted the Monster, though one possibility is sound. When Nancy ventured into the Upside Down, the Monster was unaware of her presence until she stepped on a tree root, making a loud snapping sound. Additionally, it could have been drawn to the sound of Will's radio. It has also been speculated that the Monster was attracted to electricity and other large sources of energy. This could be justified by the large amount of Christmas lights Joyce had in her house, and possibly explain why the Monster was "reaching out" to Eleven.
The Monster was hostile every time it was encountered, immediately going after whatever alerted its attention. It also appeared to use brute strength, rather than intelligence, to get through obstacles; the only exception to this being when it supposedly used telekinesis to unlock the Byers' front door. For an unknown reason, the Monster preferred to hunt in close proximity to Mirkwood, with the Middle School being the furthest it ever traveled in pursuit of prey. After catching its victim, the Monster would take them to the alternate version of the public library, where it had built something resembling a nest.
It is currently unknown whether or not the Monster has any connection to the slugs or the tendril that was down Will's throat. They could be different stages in the Monster's life cycle, or perhaps part of a larger organism that the Monster had to gather food for.
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.
- 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." They went through several early concepts, some of which were "more monstrous in shape, even more asymmetrical." They were ultimately inspired by nature when they came up with the petaled mouth in the first week, a design which really resonated with the Duffers.
The creature designs of H.R. Giger, Clive Barker, Guillermo del Toro and Masahiro Ito were influential to the monster’s appearance because of their "strangeness."
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."
- 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 presumably use telekinesis, it only exercises this ability once.
- It is possible that it used telekinesis to pull in the deer, however there is no definitive evidence to prove this.
- While the Monster exclusively hunted at night, Nancy saw it at daytime as she looked for Barb. This was simply done for storytelling purposes.
- The Monster was killed in Mr. Clarke's classroom.
- It was hinted that the falling ash-like substance present in the Upside Down has 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 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 "Stranger Things episode 6: How the Duffer Brothers created their monster" Entertainment Weekly. July 20, 2016
- ↑ 3.0 3.1 3.2 "Exclusive ‘Stranger Things’ Art Reveals ‘Upside Down’ Secrets and Barb’s Alternate End" Screencrush. August 22, 2016.
- ↑ "‘Stranger Things’ Finale: Duffer Brothers Talk Cliffhangers, Death and Season 2" Variety. July 18, 2016.