WARNING: This article contains spoilers for the second season of Stranger Things, streaming now on Netflix.
When viewers got their first look at Stranger Things' second season, what stood out most was the eerily Lovecraftian entity looming over Hawkins, Indiana. Of course, the red sky and lightning made it all the more ominous, exciting fans eager to see how the Duffer Brothers would upstage the Demogorgon. Many speculated the monster was the Thessalhydra from Dungeons & Dragons, foreshadowed by the game played in the closing scenes of the Season 1 finale.
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That D&D monster was an acid-spewing, eight-headed hydra, with each of those heads centered around a mouth containing jagged teeth. It also had a tail equipped with pincers to grab victims and feed into its mouth, but by design alone, it bore no resemblance to what we witnessed in the trailers, which was a towering multi-limbed creature that looked like a giant spider. Will Byers (Noah Schnapp), who spent much of the first season as a prisoner of the Upside Down, was the only one who could see the entity, which he called the Shadow Monster because its body consisted of, well, shadow and smoke.
However, as Season 2 neared its climax, Dustin Henderson (Gaten Matarazzo) dived into the Dungeons & Dragons Monsters Manual to rebrand it as the Mind Flayer because, unlike the Thessalhydra's physical approach to killing its enemies, this creature was more cerebral and focused on mental attacks. That said, to dissect exactly what this was that tormented Will and Hawkins, we have to go a few layers deeper, because by the time Season 2 wrapped, we'd come to discover it was so much more than a monster.
A Biological Beast
The Shadow Monster was a sentient being with an intelligence that could control creatures from the Upside Down. When Will returned from that dark mirror universe at the end of Season 1, he coughed up slugs and was plagued by visions of the destruction he Shadow Monster wanted to inflict. Dr. Sam Owens (Paul Reiser), who took over from Dr. Brenner (Matthew Modine) as head of Hawkins National Laboratory, oversaw Will's recovery, and his diagnosis played a big part in determining what this thing truly was.
Owens revealed that after returning from the Upside Down, Will wasn't just connected to it, he was infected by it. The doctor also divulged to Will's mom, Joyce (Winona Ryder), and Chief Jim Hopper (David Harbour) that this virus was quickly spreading through the boy's mind and body, confirmed through a series of brain scans. The multiple seizures Will suffered throughout the season was a side effect of the virus as it transported him to and from the Upside Down. That was happening because the Shadow Monster was trying to catch him in that dark dimension in order to prepare him as a vessel for something much more sinister, and also to make the infection permanent.