The Spider Monster was the main antagonist of the third season of Stranger Things.
It was created from the melted remains of the Flayed to serve as a physical body for the part of the Mind Flayer that had been trapped in Hawkins following the closure of the Gate.
The monstrosity was essentially an enormous weapon the Mind Flayer had created to kill El Hopper (and eventually her friends), and therefore eliminate the only thing standing in his way from annihilating the human race and taking over the Earth.
A year following Will Byers’ rescue from the Upside Down, the Mind Flayer began stalking him in a series of visions, culminating in him possessing Will through attaching a part of himself to the boy. This particulate served as a link between the Mind Flayer and Will, making him a part of the entity’s Hive Mind.
The Mind Flayer then began using Will as a means to spy on Hawkins National Laboratory so that he could eradicate the scientists who were keeping the Upside Down’s biological growth at bay. However, this connection also allowed Will to learn the entity’s weakness.
When it was discovered the Mind Flayer was vulnerable to heat, Joyce, Jonathan, and Nancy strapped Will down and surrounded him with numerous space heaters, exorcising the particulate from Will’s body which subsequently flew up into the sky. When Eleven closed the Gate, the particulate did not die like the Demodogs and other hosts but rather entered a dormant state.
In the summer of 1985, a secret Russian operation located beneath Hawkins’ newly-built Starcourt Mall successfully re-opened the Gate into the Upside Down using the Key. The particulate’s link with the Mind Flayer was re-established, reviving in Brimborn Steel Works where it immediately began to possess a horde of rats.
Mind Flayer Body
The Mind Flayer then began constructing a physical body for himself out of the rats’ remains, with the ultimate goal to kill El and her friends as revenge for closing the Gate the year prior. To achieve this, he realized he needed to find larger victims, particularly humans.
Monster First Host
His first host was Billy Hargrove, whom he then used to kidnap Heather Holloway, possessing her as well. Over the span of three days, several more Hawkins residents became hosts to the Mind Flayer, among them being Heather’s parents, Tom and Janet Holloway, as well as Doris Driscoll and Bruce Lowe. These hosts would later be dubbed “the Flayed”.
After discovering that the Mind Flayer had returned, the Party along with Nancy and Jonathan decided to use Doris as a way to find the location where the creature’s victims were being flayed. When Nancy and Jonathan went to see Doris at the hospital, they were instead met by Tom and Bruce, who proceeded to attack them.
After they had been killed in the ensuing fight, Bruce’s and Tom’s bodies melted and combined to form a grotesque monster. After the creature had been thrown out of a window by Eleven, it escaped into the sewers where it was absorbed into the Spider Monster’s body.
On July 4, the Mind Flayer finally enacted his plan; after learning Eleven’s location by using Billy as a decoy, the Mind Flayer summoned all of the Flayed to the steel mill where they were melted and absorbed into the proxy body, allowing it to grow to a massive size.
Once complete, the creature made its way to Hopper’s cabin and proceeded to attack the residence. In its assault, the creature managed to grab El by the leg with one of its tentacles, injuring her in the process.
Once the tentacle had been severed, El split the Spider Monster’s head in half, temporarily incapacitating it and allowing the group to escape.
Due to a small piece of the creature still lodged under El’s skin, the Mind Flayer was able to pursue her to the Starcourt Mall. The Mind Flayer was distracted and lured away from the mall as El, Mike, and Max evaded the creature by escaping through the mall’s back entrance. However, they were intercepted by Billy, alerting the Mind Flayer to El’s location.
Once he returned to the mall, Billy brought El to the creature to be killed. Just before he could do so, the creature was pelted with fireworks. When the fireworks were depleted, the Mind Flayer tried again, only to be stopped by Billy, who had returned to his real self.
The Mind Flayer retaliated by stabbing Billy with his tentacles, mortally wounding him. Immediately after, the Key was destroyed, thus closing the Gate and severing the Spider Monster’s link with the Mind Flayer.
Having been an amalgam of human and rodent remains, the Spider Monster’s appearance was vile, to say the least. The surface of the creature was especially irregular, having been an assemblage of bodily tissues with bones belonging to the creature’s victims jutting out from various parts of its body.
Overall, the monster’s form mirrored the Mind Flayer’s real form, possessing an elongated head and six legs that branched off to form several appendages that were used for mobility.
The creature was equipped with several prehensile tentacles that it used for offensive attacks, one of which could extend from the monster’s mouth. The maw of the creature was filled with circular rows of jagged “teeth”. While not as massive as the Mind Flayer himself, it was still exceptionally large, towering at around 30 feet tall.