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.
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.
In episode 7, Vecna is revealed to be Henry Creel (played as a youngster by Raphael Luce), the son of Victor Creel (Robert Englund) and his wife Virginia (Tyner Rushing) shown in the 1950s flashbacks.
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.
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.
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.
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.
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).
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.
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.
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 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 seasons 2 and 3, when the gate was reopened, the Mind Flayer accumulated the Upside Down's many casualties from Hawkins. After seven years of being captive in the Upside Down, Vecna finally began taking his own victims again in Stranger Things season 4's 1986 timeline.
The character is a longtime D&D villain, one who used his abilities as a mage to turn himself into a lich (an undead magician) in order to avoid a mortal death. Despite achieving immortality, Vecna's physical body was almost completely destroyed by his follower, Kas.
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.
(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.
While falling into the dark dimension, One/Henry is struck by lightning, disfiguring him.
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.
Motivated by a cruel and misanthropic philosophy, Vecna targets particularly traumatized, mentally ill or insecure individuals.
His method of killing is ritualized. Vecna started twisting bodies and removing eyes before imprisonment in the UD to fuel its own individual power. It takes the eyes to help hold the souls in darkness and because it did not wish to be observed.
"The reason why Vecna only has a single eye and hand is due to a betrayal by Kas", who used the sword against his former master. "In the third edition of Dungeons & Dragons, the Sword of Kas is a +6 unholy keen vorpal longsword that grants a +10 modifier to the Strength score of its wielder".
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.
In the final episode of Stranger Things 4 Volume 1, it's revealed that Vecna isn't native to the Upside Down, but was banished there by Eleven after he gruesomely massacred the Hawkins National Lab.