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.
One powerful reveal shows that Vecna was a boy called Henry Creel, who discovered he had powers such as telekinesis and a form of mind control, which he used to kill his family. His father, Victor Creel (played by horror icon Robert Englund), was blamed.
According to the helpful folks over at the Dungeons & Dragons Forgotten Realms fandom wikipedia, Vecna was once a human king who became a lich (a powerful undead wizard). Over time, Vecna grew in power enough to ascend to godhood.
A Troubled Henry Creel Moves to Hawkins. Vecna was not always a hideous beast of the Upside Down. He was once a troubled child in the real world named Henry Creel. Henry's “problems” caused his family to move to Hawkins in 1959.
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.
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).
Before coming to the Hawkins National Lab, Venca had a classic villain origin story — he became obsessed with black widows, tortured his family, and brutally killed his mother and sister pinning it on his father.
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.
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'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.
Vecna was born as a human, centuries ago as a member of the untouchable caste in the Flan city of Fleeth on Oerth. He was initially trained by his mother, Mazzel, in the art of magic, before she was executed by the government of Fleeth for practicing witchcraft.
Stranger Things' Vecna is inherently evil and doesn't seem to have a personality outside his desire to eradicate living things (something that isn't necessarily a bad thing.) As Henry Creel, he has always looked down on people and deemed that he is of a superior form.
"Vecna really is a creature that has mutated in the last however many couple of decades during his time in the Upside Down," prosthetics designer Barrie Gower said. "This is where he has become more overtaken by the Upside Down world and become pretty much possessed and mutated."
Vecna hated humanity, believing human beings had imposed an artificial order upon the natural world.
"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."
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.
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.
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.
(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.
Kind-hearted cheerleader Chrissy Cunningham (Grace Van Dien) is the first character to be cursed by Vecna, and she dies in season 4, episode 1, "Chapter One: The Hellfire Club." After feeling ill, Chrissy sees the clock and begins picturing her mother, who has always been cruel, saying harsh things about her physical ...
Vecna's curse refers to Vecna's predominant method of murder and torture. Using his supernatural psychokinetic abilities, Vecna can probe and influence the minds of others. Motivated by a cruel and misanthropic philosophy, Vecna targets particularly traumatized, mentally ill or insecure individuals.
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.
What is the significance of 4 for Vecna? The new season shows that the four chimes of the grandfather clock symbolize the four victims Vecna needs to claim in order to enter our dimension and take over humanity.
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.