Eleven has a memory during this episode and realizes that 001/Henry Creel (Jamie Campbell Bower) wants her to join him on his evil journey, and she banished him to the Upside Down.
When Eleven realized what Henry had done, she summoned an enormous amount of power and defeated the older man. Her psionic abilities were so advanced (even at the age of about 7 years old) that she opened a gateway between dimensions and banished Henry into the Upside Down.
Indeed, Stranger Things season 2 revealed that Eleven wasn't dead, and was instead sent to the Upside Down. She escaped from the other dimension and came back to the human world a couple of hours later, but by then, the school was surrounded by policemen and the kids were being interrogated.
So, 1979 is when Eleven confronted 001 a.k.a. Henry Creel in the Rainbow Room and sent him through to the Upside Down, turning him into Vecna in the process—and effectively putting in motion all the events of the seasons we've seen so far, from the Mind Flayer of Season 3 to the four murders of Season 4.
One manipulated Eleven into plotting her own escape, which also involved getting her to remove the device in his neck. (Henry, however, did not tell Eleven the truth when he said Brenner wanted her dead.
As well as befriending Eleven in order to gain her trust so she would remove his Soteria implant, we learn in the final episode that 001 initially spared El because he wanted her to join him. He viewed her as an ally rather than an enemy because they were "alike".
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).
Realizing he had tremendous psychic power, he haunted his family with visions before ultimately killing most of them. His father was framed for the murders and locked away in a mental hospital as a disturbed serial killer. Henry then found himself in the care of Brenner, who decided he wanted more kids like the boy.
Eleven has a memory during this episode and realizes that 001/Henry Creel (Jamie Campbell Bower) wants her to join him on his evil journey, and she banished him to the Upside Down. 001 then transformed into Vecna, the horrifying Stranger Things villain.
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.
Henry told Eleven that the Soteria in his neck was a tracking device that also 'weakened' him. Wishing to help him, Eleven was tricked into ripping the device out with her abilities.
“I don't think that she created the Upside Down,” Brown told Variety in June. “No, I think that it was always there, I think she just created a gate to it which no one could before.” Brown is likely right in saying that no one created the Upside Down and that the parallel dimension has always existed.
"I walked up to her [as Vecna] and she burst into tears. She wouldn't look at me and she was visibly just disgusted by the whole thing... After she cried and I made it obvious that she knew it was me, one of the things she said was that, 'I knew it was you when I could smell cigarettes,' because I'm a smoker."
What does Vecna want with El? 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.
One/Vecna had his powers blocked and was working as an aide at Hawkins National Lab where he befriended Eleven. Not knowing that One was evil, Eleven restored his powers which he used to gruesomely massacre the entire lab.
His first victim was Chrissy Cunningham. Vecna targeted Chrissy by using her depression and eating disorder against her in his visions. Chrissy was killed in Eddie Munson's trailer, and the first gate was opened on the ceiling.
Joe Manganiello's character Arkhan fought against Vecna with the adventuring party Vox Machina in the web series Critical Role. He then stole the Hand of Vecna and replaced his own hand with the artifact.
However, within the red smoke of Vecna's Mind Lair, it wasn't the song itself that saved her, but the memories it conjured up. What broke through to Max was the happy, loving memories of her friends, particularly Lucas and Eleven, and these were what truly allowed her to break free of Vecna's grasp and escape.
In simple words, Vecna needs a total of four gates to take over Hawkins and rule it. Now, you must be thinking, why is Vecna killing people if he can take over the complete Hawkins? Well, in one of the previous episodes, it was revealed that with each person Vecna kills, he creates an opening to the real world.
"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."
As Vecna, he uses these mental abilities as a way to psychologically weaken and lure in his victims – much in a way that Pennywise, Freddie Kruger, and Pinhead do. (All of which have been cited by show creators the Duffer brothers as inspirations for the villain.) But he does have a surprising weakness: music.
In his dying moments, Brenner deactivates Eleven's collar. Ultimately, on some deeply messed up and fundamentally irredeemable level, he did care for her. Although Brenner was an abusive monster, it's still traumatic for Eleven to see her “papa” die.
Turns out he not only survived the family massacre, he caused it. Henry was put into psychiatric care after his family was killed and Dr. Brenner, alert to Henry's powers, made him 001, the very first of the kids experimented on.
Mike and Eleven have been the couple to root for from the start. After everything they've been through, Season 4 proved that not even a long-distance relationship could dent their relationship.
However, Bower found Brown's fear of Vecna somewhat amusing and mentioned that he was talking to her inside his mind saying, “No, Jamie's dead. It's just Vecna.” Brown ends it by saying the fear was genuine and that she didn't have to act because she was actually terrified.