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.
Nancy sees an image of Barb's dead body in her vision of the Upside Down.
Who is Vecna really? We find out in episode 7 that just like how everything in the Upside Down is connected, Vecna is also deeply connected to Hawkins and Eleven. Nancy (who is still in the Upside Down with Steve) ends up under Vecna's control, and she starts seeing visions about Barb's death.
The following day, she noticed that Barb was not at school and grew increasingly worried when neither her friends nor Barb's mother had seen her. Nancy went back to Steve's house to look for her and was startled by a creature in the forest.
Vecna then shows Nancy a vision of the future, where Hawkins is torn apart by rifts, before releasing her. A traumatized Nancy relays Vecna's vision to the rest of the group, who resolve to kill Vecna that night.
However, either she didn't have a great aim or she was trying to play it safe because she didn't hit him in the face even once, and that may be the reason why Vecna is still alive. Obviously, Nancy was trying to play it safe. She even cut off the muzzle of her shotgun so she could have the best odds.
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.
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.
Nancy: She Didn't Love Him
Nancy and Steve never shared the same connection as Nancy and Johnathan and were mismatched in regards to their personalities. Nancy accepts that she doesn't love Steve in season two, but this leaves the fallen jock heartbroken.
One explanation for why time in the Upside Down is frozen, rather than flowing ahead to 1986 like in the real world, could be the impact of Eleven opening that first gate. The outburst of power she used and the deep emotions that were running through her were enough to shock the Upside Down into time standing still.
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).
It seems that the reason why Vecna wasn't killing for the seven years after the Hawkins Lab massacre was because he was waiting to become more powerful as the Mind Flayer and Demogorgons took more victims, as he was likely weakened after being thrown into the dimension.
The Duffer Brothers had Vecna planned from the start, which is how they were able to tie his character into earlier seasons. Stranger Things revealed Vecna was also behind the Mind Flayer that appeared in season 2.
Nancy Cared About Steve More
Nancy chose to remain with Steve rather than accompanying Barb home, despite her friend having hurt herself. It could be argued that Nancy cared about her boyfriend more, as she disregarded the feelings of her friend.
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.
Nancy got upset, walked away and said: “I can't believe you right now.” Steve did get into trouble with his parents but went to Nancy's home to apologize the day after.
Steve Harrington has not been able to find love after Nancy. Here's why the Stranger Things character should let the past stay in the past.
Let's be honest: what Nancy let happen with Jonathan was basically romantic cheating. As such, Steve and Jonathan end up getting into a fight, and it appears that Nancy leans towards choosing Jonathan. That said, the fight was exactly what Steve needed to start his character development.
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.
Thinking back to Chrissy and thinking of their friends that are still counting on them, Eddie cuts the rope back to his trailer. He tells Dustin that he is going to buy more time for their friends and exits the trailer, jumping on his bike and leading the Demobats on a chase.
Barb sits outside near the pool and the Demogorgon grabs her, and before she knows it, in episode 3 "Holly, Jolly," she's in the Upside Down. The Demogorgon then kills her brutally. One of the scariest Stranger Things creatures, the Demogorgon sees Barb, knows exactly what it wants with her, and spares her no mercy.
Next up, the viewers go on to theorise that One/Vecna is actually Eleven's dad. Yep, told you it was wild. The theory claims that Brenner wanted to create a child with stronger powers, so he waited until One was old enough to become a father.
Vecna's Hand
This is a reference to the D&D character Vecna, for whom this Vecna is named; the D&D Vecna has a monstrous hand infused with dark magic. The design of Vecna's hand is also a nod to Freddy Krueger, the Nightmare on Elm Street villain who had a huge influence on this season in general.
In his twisted mind, Vecna believes that he his helping the world by wiping out humans, as a "predator, but for good." When Vecna asks for El to join him, it's because he also sees her as a predator, "better" than humans. He offers her his path as a sort of freedom, as they reshaped the world in their image.