Vecna is One, who is Henry Creel. One, previously known as Henry Creel, ultimately became Vecna after Eleven defeated him at Hawkins Lab. That also explains a few other things, like Henry's fascination with spiders and Vecna's web-like, vine-y appearance. (The D&D Vecna is also said to sit on a Spider Throne.)
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.
Vecna was Dr. Brenner's "Number One," inspiration for the experiments he conducted with Eleven and other children like her. Eleven unwittingly freed him, and he massacred the other Hawkins children; he was unable to defeat Eleven herself, however, and she banished him to the Upside Down.
So, Vecna is actually 'Experiment 001' from the Hawkins Lab, case closed. Well, not quite! Not only is Vecna the new alter-ego of 001 after being banished to the Upside Down, but he is also the son of Victor Creel.
Provided he actually ages in the Upside Down, Vecna could be in his late 30s in 1986, the year that Stranger Things season 4, volume 1 takes place. Vecna became a monster seven years prior, when Eleven defeated the adult Henry Creel, a.k.a. Number One, at Hawkins Lab and he became banished to 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.
From the moment that he is born, 001 is a strange and odd child, and he seems to have powers from the very beginning of his life. When he begins living at the Hawkins Lab and Dr. Martin Brenner (Matthew Modine) experiments on him, it's clear that he is different from the other children.
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.
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.
Then, as he was falling through the Upside Down, One became the creature Vecna after lightning struck him in such a way that it changed his skin and made him into the monster he is now.
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.
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.
At his empire's height, Vecna was betrayed and destroyed by his most trusted lieutenant, a vampire called Kas the Bloody-Handed, using a magical sword that Vecna himself had crafted for him, now known as the Sword of Kas.
Brown is likely right in saying that no one created the Upside Down and that the parallel dimension has always existed. And if the boys' science teacher was right in saying there are “infinite variations” of our world in alternate dimensions, there could even be several versions of the Upside Down out there.
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).
She opened a portal to the Upside Down, which sucked Henry Creel in. There the dark mirror world ripped apart his skin as it began to remake him into something sinister and twisted. But it was a transformation Henry Creel welcomed.
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 seemingly acquired Eleven's ability to influence minds from across dimensions after a piece of the Mind Flayer's proxy body bit Eleven in July 1985, a being that the possessed Billy said was “made for” her.
Looking at the Kas angle, Eleven did form a similar Kas-Vecna alliance in her past before betraying Stranger Things' Vecna and sending him to the Upside Down.
If the eyes and the right hand weren't enough to convince you already, it's confirmed that Vecna was once human – and he has a very strong connection to Eleven and the Hawkins Lab.
As Sink put it: “You kind of learn in the season that he targets people that are in a real [bad] place, and Max is obviously one of those people. Just with everything with Billy, so she was kind of like the perfect target.
Jamie Campbell Bower plays 001 in Stranger Things 4.
He gets punished with electrocution after telling Eleven that Dr. Brenner lied to her about her mother dying during childbirth.
Oh Il-nam (Korean: 오일남), also referred to as the Host or Player 001, is the main antagonist of the first season of the South Korean Netflix drama Squid Game.
The boy, who was named and tattooed “001,” was the first of Brenner's experiments. When Eleven defeated him, he was cast into the Upside Down and his body was mutilated by the warped lighting as he fell.