Re-watching the episode where
Vecna was the one who originally kidnapped Will in season 1.
He was spared from any possessions or abductions in Stranger Things seasons 3 and 4, but the show has confirmed that Will is still connected to the Upside Down's master, Vecna.
In season 4, it's revealed that Will's connection is to Vecna, who created the Mind Flayer after being banished to the Upside Down by Eleven (Millie Bobby Brown).
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.
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.
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.
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).
At the same time, Nancy, Steve and Robin reach Vecna's body in the Upside Down and throw multiple Molotov cocktails at him before Nancy shoots him, sending his blazing body flying out the window of the Upside Down's Creel House.
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.
Originally appearing in the Greyhawk campaign setting, Vecna was described as a powerful wizard who became a lich. He was eventually destroyed, and his left hand and left eye were the only parts of his body to survive.
Though Vecna is now more powerful than Eleven, at the time she managed to escape through a psychic joust. Kali's escape was far more subtle, happening before the massacre could even take place. The series implies that Kali was able to escape by using her abilities.
After a certain period of time, Vecna will finally kill the victim by placing them into a comatose-like state, in which they will be unable to respond to outside stimuli.
So the connection between Will and Vec — Can I call Vecna/Henry/One just “Vec”? — is one of sheer chance. They are connected because Will was the first victim. That's it.
Ultimately, it is revealed in Stranger Things season 4's finale that the reason why Vecna was killing Hawkins teenagers was to create a massive gate that merges the Right Side Up with the Upside Down - and he specifically needed four bodies to do so.
The most obvious answer is that he is challenging Eleven for a fight. That's the reason he wants Nancy to share her visions with Eleven. Moreover, Max observes that Vecna's clock always chimes four times which indicates the four kills he wants. Then she adds up that he has been telling them their plan this whole time.
Vecna prays on those dealing with trauma, which is why Max, who recently lost her brother, Billy, was the perfect victim. While she initially got away from him with help from her friends and her favorite song, she wasn't as lucky at the end of the season when Vecna captured her again.
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 the D&D World of Greyhawk campaign, Vecna was almost fully defeated by his trusted lieutenant, Kas the Bloody-Handed. Arming himself with a sword and a spiked shield, Kas betrayed Vecna and attempted to slay him, taking Vecna out of his physical body before the evil lich eventually returned.
Trauma fuels him
Also in episode 8 of Season 4 named “Papa,” Dr. Brenner stated that Vecna consumes everything about his victims.
"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.
Trivia. The Demogorgon Dungeons & Dragons figure, used by Eleven to symbolize the Monster. The Demogorgon received its nickname from Eleven using the Demogorgon game piece from a Dungeons & Dragons set to show that Will was hiding from the creature in her vision of The Upside Down.
Sharing more about Eleven's real father Andrew Rich could be a great way to wrap up the story.