- Vecna was the one who originally kidnapped Will in season 1.
These abilities would manifest as a strange sensation on the back of his neck, whenever an Upside Down monster was close. 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).
Those who spot a grandfather clock and hear its chime are thought to be cursed by Vecna, but if you rewind to season one, Will himself falls victim to the sound, which could explain why he was taken in the first place.
In the debut episode of Stranger Things season 1, Will was taken into the Upside Down by the Demogorgon. The details surrounding his abduction were never clarified in the Netflix series but the Stranger Things comic miniseries published by Dark Horse Comics filled in many blanks.
Re-watching the episode where Will Byers goes missing, there is a lot of evidence suggesting that it was Vecna who abducted Will and trapped him in the upside down. Many Stranger Things fans who re-watched the scene pointed out that they could hear the chimes of the grandfather clock.
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.
Vecna's first big credited kill was Chrissy Cunningham (Grace Van Dien). He followed that murder with canonically terrible driver Fred Benson (Logan Riley Bruner) and basketball star Patrick McKinney (Myles Truitt).
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.
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.
They are connected because Will was the first victim. That's it. Eleven is Vecna's true foil, an anti-Vecna, if you will. We learned a lot about Eleven's connection to Vecna and how their brutal fight in Hawkins Lab manifested the Upside Down.
In her rage, Eleven looks to disintegrate One, but instead sends him through to the Upside Down, opening her first gate to that other dimension. One bursts into flames as he is struck by beams of crackling energy, and ultimately becomes Vecna.
One of the theories could be that Will has some supernatural powers that caused the monster to be lured to him even though he wasn't injured. That could also be the reason he could communicate with his mother using the lights.
But eventually the Demogorgon found Will and dragged him to the library, where he was harvesting and storing the bodies of humans and animals. In order to rescue him, Will's mom and Hopper enter the Upside Down through the gate that Eleven unintentionally opened.
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.
Vecna was the one who originally kidnapped Will in season 1.
"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.
After taunting her with incessant chiming from a grandfather clock and appearing before her in hallucinations, Vecna eventually kills her in an incredibly gruesome way. First, she went into a trance with her eyes rolling into her head, then she began to levitate before her eyes were removed and bones were snapped.
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.
Many theorized she was—spoiler alert—still alive and would make it out of the Upside Down, and they had pretty convincing evidence to back the idea up. But unfortunately, Justice for Barb isn't going to happen.
Brenner. The episode culminates in Eleven unknowingly freeing One from Brenner's control, allowing him to unleash his true power on the staff and children in the lab, killing everyone except for Eleven and Dr. Brenner, who is only knocked unconscious.
At the height of his empire Vecna was betrayed by Kas. Kas managed to destroy the Archlich before his own death, with the Sword of Kas, a lethal blade of the Undying King's own make, leaving only Vecna's left hand and left eye behind. Vecna eventually returned and by 581, had risen to godhood.
Sharing more about Eleven's real father Andrew Rich could be a great way to wrap up the story.