Nancy sees an image of Barb's dead body in her vision of the Upside Down.
In the Upside Down, Steve is frantically shaking Nancy, who has gone into a trance: Vecna's curse. In Vecna's illusion, Nancy sees Barb in the pool, her body decomposing, a slimy vine slithering out of her open mouth. Blood starts to pour down the side of the pool while Nancy screams.
Nancy finds Barb's car and sees the creature in the woods. Joyce establishes a code with Will using the lights, which he uses to tell her that he is alive but unsafe. He tells Joyce to run as a creature begins to climb through her wall.
Nancy was then later cursed by Vecna. Using past traumas to break down his victims, Vecna brought Nancy to the pool where Barb was killed by the Demogorgon and revealed her mangled corpse. Vecna then taunted Nancy with her own feelings of guilt - that she was responsible for Barb's death and had forgotten about her.
After Nancy Wheeler entered the Upside Down through a temporary portal, she observed the Demogorgon as it feasted on the deer. The Demogorgon noticed her, chasing her until she returned through the portal.
When Nancy describes her visions, she talks about a creepy monster with a gaping mouth. It doesn't sound like one we've seen this one in the Upside Down before. So what is it? Our theory is that it is the Thessalhydra.
Barb's death implied that whatever the slug does didn't take, so the Mind Flayer reached out to Will instead. It could be that the Mind Flayer needed a younger host, or Barb may have simply died while the Demogorgon was attempting to infect her.
It's a surprising and emotional moment and a testament to the show's smart storytelling that such a minor character could make fans feel so sad. Barb is incredibly innocent and dies because she was in the wrong place at the wrong time and trying to be the best friend she could be.
Vecna, who preys on the shame of his victims, taps into Nancy's guilt and grief over Barb's death. Nancy sees an image of Barb's dead body in her vision of the Upside Down.
In episode 7, Vecna is revealed to be Henry Creel (played as a youngster by Raphael Luce), the son of Victor Creel (Robert Englund) and his wife Virginia (Tyner Rushing) shown in the 1950s flashbacks.
The password to enter is "Radagast," the name of a wizard in J.R.R. Tolkien's The Lord of the Rings.
After dark, they found a wounded deer. They agreed to kill it out of mercy, but before they could it was suddenly dragged away. Following the blood trail, Nancy entered a hole in a tree and found herself in a different dimension. She saw the Demogorgon feeding on the deer and ran away as it gave chase.
Hopper spent a relentless search through the trailer, breaking and ripping things apart, looking everywhere for a bug that the agents would use to eavesdrop on him, until he discovered it in a light.
"Vecna seems to be actually intrigued by Nancy being able to figure him out, to be able to get this close to the truth... I think this is more Vecna's narcissistic traits that pull him to Nancy, why he wants to give her this information."
While intending to get his revenge on Eleven and Brenner, Vecna also goes after victims who have dealt with traumatizing experiences in the past (reminiscent of his childhood and his time at the Hawkins lab).
The theory is that Karen Wheeler might be Vecna's sister, Alice Creel. As we know, Vecna is actually a man of many names, including 001, Peter Ballard and Henry Creel. We saw Henry and Alice together as children briefly in a flashback and it is believed that Henry killed his sister along with his mother.
He dies in Dustin's arms in the Upside Down, and his body is never recovered. Days later, Dustin tells Eddie's uncle that he died a hero, fighting for the town despite the fact that everyone in it wrongly hated him.
Many fans didn't like how Nancy abandoned her best friend to hook up with Steve, which resulted in the Demogorgon ambushing and abducting her. Sadly, Barb was taken to the Upside Down, dying there, which Nancy eventually saw herself.
The teenager was dragged into the Upside Down and quickly killed by the creature. Nancy later spent time searching for her friend until the truth about her death was unearthed. Seeing as Barb served as a symbol of innocence, viewers were upset about her sudden death.
However, when Barbara goes missing, Nancy jumps back into the investigation, also feeling guilty because Barbara disappeared while Nancy was with Steve. She aligns herself with Jonathan and they discover Barbara has been taken to an alternate dimension called The Upside-Down by a creature called The Demogorgon.
Marsha and her husband decided to hire Murray Bauman to investigate Barb's disappearance; although this was a costly undertaking, even forcing them to sell their home, they viewed it as justifiable.
Cynthia Barrett, Barb's Mom on “Stranger Things”
The theory predicts that Vecna wants to regain his human form, and he will use Will to do so in Season 5. Knowing that Vecna singled out Will in Season 1, it's a reasonable theory, but it might go too far. After all, Vecna already has a human form, and he has vowed to eliminate humanity.
The slug Will coughed up is a baby Demogorgon. Though the actors were a little hesitant to confirm this on the after-show, it seems pretty apparent that the slug Will coughed up at the end of “Stranger Things” Season 1 was Demogorgon spawn, or at least a baby Demodog.
He hid in the shed but the Demogorgon caught him and took him to the Upside Down. Now, some fans believe Will hurt himself enough to bleed even if slightly when he crashed his bike, and that's why the Demogorgon went after him.