Dustin (played by Gaten Matarazzo) named the villain Vecna after noticing similarities between his powers and those of a character in the game Dungeons and Dragons.
The final scene of Volume 1 shows a shot of Vecna, and a close up of the 001 tattoo on his arm, confirming that Henry Creel, a.k.a. Number One, became Vecna.
Original Dungeons & Dragons
The name Vecna was an anagram of Vance, the surname of Jack Vance, the fantasy author whose works inspired the magic system used in Dungeons & Dragons.
He reveals what looks to be a tracking device implanted in his neck. As a thanks, Eleven uses her powers to remove the chip, which leads the orderly to reveal his true identity as 001. 001 was born Henry Creel, the son of Victor Creel (played by A Nightmare on Elm Street's Robert Englund).
In season 4 episode 7, it's revealed that Henry caused the massacre at the lab in 1979. Eleven (Millie Bobby Brown) fought and stopped him, but inadvertently sent him to the Upside Down, where he became Vecna.
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).
"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."
The episode then cuts to Vecna in 1980s Upside Down, zooming in on his arm, where a tentacle slides away to uncover 001 tattooed on his wrist. So Vecna is 001, who is also Victor Creel's son.
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).
She Doesn't Have a Great Aim
After the Hawkins team develops a plan to defeat Vecna, Nancy is in charge to blow his head up with a shotgun. 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.
Vecna did not just kill his victims he haunted them with their traumatic past before completing the deed but his 1986 victims were the only ones who were not tortured before their death. The main reason could be that Vecna was powerless during that period and killing them was the only way he could escape.
Vecna's obsession with time appears to be linked to his hatred of humanity. He views time as a human imposition on the natural world; an attempt to impose order on nature.
Vecna was the original child studied by Dr. Brenner and the son of Victor Creel. He's the infamous Number One, but had his powers blocked and worked as an aide in the Hawkins National Lab.
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.
Played by Jamie Campbell Bower, each prosthetic was glued to his skin with medical adhesive by prosthetics designer Barrie Gower and his team. For the finishing touch, Vecna was covered in "glossy slime to make him really, really glossy and wet looking," according to Gower.
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.
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.
What was Chrissy's Trauma? Chrissy's life appears to be free from flaws on the surface, but in reality, she is struggling terribly to conceal her depression and self-image stemming. She was struggling with these issues, which resulted in her developing an eating disorder due to her mother's verbally abusive comments.
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.
Seeing Stranger Things' Vecna Made Millie Bobby Brown Cry
She's a 10, but she cries when she sees Vecna? She's an Eleven.
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.
Once he takes control of the mind, he traumatizes them by showing the visions of their dark past. He takes them to unknown places and gives them a glimpse of himself. The pain of his victims doesn't end here. Every single one of his victims experiences nose bleeding and severe headaches as well.
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.
Vecna successfully stole Eleven's powers when controlling the Mind Flayer in Stranger Things' season 3 ending, which finally allowed him to emerge stronger than Eleven and enact his plan to take over Hawkins with the Upside Down.
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.