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.
As a result, Peter/Henry was banished to The Upside Down, where he would remain until he transformed into a creature called Vecna. Vecna ultimately becomes the main antagonist in season 4 of Stranger Things. Like Henry in his human form, Vecna possesses the abilities of mind control and telekinesis.
Henry Creel = Brenner's Assistant = 001 = Vecna
Then Eleven—who in this future moment discovers that she in fact does not have the blood of her siblings on her hands—defeats Henry Creel, sending him to the Upside Down where he gets blasted by lightening and burnt to a crisp and, thus, becomes Vecna. That was 1979.
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.
Originally, the season made it look like Eleven snapped and killed everyone. Later, an extended version of the scene showed that Henry Creel/001/Vecna was the real killer. Either way, the implications were simple: Dr. Brenner and Eleven were the only survivors.
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 was the one who originally kidnapped Will in season 1.
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.
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).
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.
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).
"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."
Soteria is a miniature device that, when implanted into the neck of an individual with psychokinetic abilities, inhibits that person's abilities. Henry Creel had such a device inserted into his neck by Dr. Brenner, who probably created the technology.
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.
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.
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.
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.
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.
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.
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.
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.
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.