In a fit of rage, Henry used his restored powers to brutally murder the other test subjects and workers, and attempted to do the same to Eleven. However, Eleven stood her ground and ultimately overpowered him, sending him through a gate to the Upside Down; there, he gradually transformed into the being known as Vecna.
There's only a short time between when a recognizable Bower first shows up on screen in Episode 6 to the end of Episode 7 when Eleven blasts Henry into the Upside Down, permanently disfiguring him and turning him into Vecna, but Bower makes the most of every minute of it.
The Upside Down is first expected to be exposed to the real world in 1979, during Eleven's fight with One/Henry Creel in the Rainbow Room of the Hwakins Laboratory when Eleven unintentionally opened the gate while overpowering One, sending him to the dark world, where One became Vecna.
Henry “had found his strength in a memory from his past, something that made him sad, but also angry.” For Eleven that was the memory of her mother being dragged away from her. And as she pulled all her rage and her pain from deep within her soul, Eleven not only overpowered One, she sent him to another dimension.
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.
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).
So it stands to reason that Henry inherited his powers from one of his parents. And, one thing more -- remember that Victor Creel did NOT suspect Henry of being the source of the family's problems - he thought a demon was behind it.
Curiously, he also seems obsessed with time, projecting strange visions into his victims' minds—including of the grandfather clock from his home in the Creel House. Vecna's obsession with time appears to be linked to his hatred of humanity.
Vecna's Hand
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 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. He offers her his path as a sort of freedom, as they reshaped the world in their image.
It seems that the reason why Vecna wasn't killing for the seven years after the Hawkins Lab massacre was because he was waiting to become more powerful as the Mind Flayer and Demogorgons took more victims, as he was likely weakened after being thrown into the dimension.
However, within the red smoke of Vecna's Mind Lair, it wasn't the song itself that saved her, but the memories it conjured up. What broke through to Max was the happy, loving memories of her friends, particularly Lucas and Eleven, and these were what truly allowed her to break free of Vecna's grasp and escape.
Vecna hated humanity, believing human beings had imposed an artificial order upon the natural world.
After previously having tricked Eleven into destroying the chip, Peter goes on a killing spree in the lab until a confrontation with Eleven ended in her casting him into the Upside Down - where after falling through various dimensions and sustaining various injuries, Peter was transformed into Vecna.
The Duffer Brothers had Vecna planned from the start, which is how they were able to tie his character into earlier seasons. Stranger Things revealed Vecna was also behind the Mind Flayer that appeared in season 2.
Vecna's '86 victims include Chrissy Cunningham (Grace Van Dien), who suffered from her mother's body-shaming abuse. School newspaper reporter Fred Benson (Logan Riley Bruner) fled from a deadly car accident that continues to haunt him. Patrick was targeted due to his father's abuse of him.
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.
Motivated by a cruel and misanthropic philosophy, Vecna targets particularly traumatized, mentally ill or insecure individuals.
After learning how Victor Creel was able to evade death back in 1959, Robin and Nancy deduce that playing the victim's favorite song can help them escape the trance that Vecna places them in, with music and happy memories being the only way to keep Vecna's fatal curse at bay.
Netflix Vecna has plans for world domination. Essentially, Vecna hates humanity, and wants to take over their world. He had hoped that Eleven would help him achieve this goal. But when he realised she wouldn't, Vecna instead used her.
The Creel family was a family that bought a house in Hawkins, Indiana in 1959. They experienced strange, and supernatural occurrences in their home during their residency. This subsequently resulted in a multiple homicide committed in the home.
The history of the Upside Down remains mysterious. Exactly how and why it came into existence is unclear. Dustin theorised the Upside Down had existed for thousands, if not millions of years.
Peter was a false alias given to him by Brenner. Also, Henry's character was given the name Peter when he was revealed on the Stranger Things Twitter. Yeah, it's a cover name. Would be kinda suspicious if they called him Henry Creel.
Hawkins has suffered a lot of trauma over the years - not all of which is connected to Hawkins Lab and the Upside Down - and Vecna's curse takes advantage of that. Vecna appears to carefully choose people who feel a strong sense of guilt or shame, particularly if those emotions are repressed.
In the visions of her trauma induced by Vecna, Chrissy's mother appeared both mentally and emotionally abusive towards her daughter - and it's implied her comments about Chrissy's figure caused her daughter to develop her eating disorder.