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.
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.
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.
During One's brutal attack on the lab, El defeats him by using her own powers to transport him into a dimension that we now know as the Upside Down, where he is absorbed by its power and eventually becomes Vecna.
There's been debate as to whether or not this scene implies that Eleven created the Upside Down or merely opened the first gate to it. But according to Millie Bobby Brown, the Upside Down existed before El ever meddled with it. “I don't think that she created the Upside Down,” Brown told Variety in June.
In a fit of rage, Henry used his restored powers to brutally murder the other test subjects and workers, and attempted to do the same to Eleven. However, Eleven stood her ground and ultimately overpowered him, sending him through a gate to the Upside Down; there, he gradually transformed into the being known as Vecna.
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. Brenner often lied, but his actions towards Eleven were about control and unlocking her powers, not killing her.
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 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.
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.
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.
In season 4, Peter Ballard notes that Kali was no longer at the lab in 1979. Given Kali's powers, it seems pretty self-explanatory as to how she was able to escape the lab. She either made herself invisible or was able to escape after manipulating the minds and visions of the guards.
Vecna is also a marvel of special effects wizardry, being mostly practical (giving the actors a physical presence to react to) but decorated with CGI, giving Vecna the appearance of a writhing, repulsive entity, whose tendrils seem to have a mind of their own.
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 is the primary leader of The Upside Down Hive Mind. The Mind Flayer seemed to have been connected to this Hive Mind and was possibly the leader of the Hive Mind before Vecna came and connected to himself making The Mind Flayer's Hive Mind into Vecna's Hive Mind.
If he couldn't physically leave the Upside Down, Vecna needed to use the Mind Flayer to attack Hawkins. He also needed to figure out how strong Eleven's power had gotten since their last face-off. Through Demogorgons and The Mind Flayer, Vecna began to slowly break down the barriers between the two worlds.
Will Became Connected to Vecna During His Kidnapping in the Upside Down. The assumption has always been that Will survived in the Upside Down on sheer dumb luck before being trapped in those creepy hive mind vines.
Vecna got his start as a couple of magic items
Vecna first appeared in 1976's Eldritch Wizardry, a supplement for the original Dungeon & Dragons rules. Co-author Brian Blume named two artefacts in the supplement: the Hand of Vecna and the Eye of Vecna.
Hawkins has suffered a lot of trauma over the years - not all of which is connected to Hawkins Lab and the Upside Down - and Vecna's curse takes advantage of that. Vecna appears to carefully choose people who feel a strong sense of guilt or shame, particularly if those emotions are repressed.
(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.
School newspaper reporter Fred Benson (Logan Riley Bruner) fled from a deadly car accident that continues to haunt him. Patrick was targeted due to his father's abuse of him. Finally, Vecna targets Max for her trauma from witnessing Billy's death and uses Barb's (Shannon Purser) death as a way to use Nancy.
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.
Curiously, he also seems obsessed with time, projecting strange visions into his victims' minds—including of the grandfather clock from his home in the Creel House. Vecna's obsession with time appears to be linked to his hatred of humanity.
Vecna hated humanity, believing human beings had imposed an artificial order upon the natural world.