When Will steps foot back in Hawkins, those strange feelings start to creep back. He can feel Vecna's presence, though the creature was severely hurt when Steve (Joe Keery) and Robin (Maya Hawke) set him on fire and Nancy (Natalia Dyer) popped a few shotgun rounds into him.
Furthermore, aspects like the Russians' heightened interest in Hawkins Lab, the creation of Eleven, and the coincidence that Henry Creel became so powerful once he went to Hawkins all support the idea that the town has further significance than originally thought and perhaps even that Vecna's powers were caused by ...
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.
Near the end of Stranger Things Volume 1, Dustin speculated that Vecna is the Mind Flayer's “five-star general,” but the truth is nowhere near that simple. From the information we got during the Stranger Things 4 finale, we can conclude that Henry Creel is the one calling the shots; in fact, Henry is the Mind Flayer.
Nancy later led the team back into the Upside Down with the purpose of killing Vecna as Max, Lucas, and Erica lured him in, and Nancy, Robin, and Steve managed to burn Vecna, with Nancy shooting him several times, and though they weakened the creature, he's still alive.
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).
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.
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.
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.
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.
Vecna holds off Eleven long enough to brutally attack Max in his signature style—she levitates, her eyes go white and bleed, and he snaps her limbs. Eleven regains her strength and pushes Vecna off before he can complete the job, but the damage is already done.
He believed that the construct of time and monotony needed to be destroyed. So, in his mind, by destroying humanity, he was doing it favor. That's what makes him so evil.
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.
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.
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. Vowing revenge, Vecna eventually assumed a mastery of the dark arts achieved by no mortal before or since.
"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."
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.
Though Vecna is now more powerful than Eleven, at the time she managed to escape through a psychic joust. Kali's escape was far more subtle, happening before the massacre could even take place. The series implies that Kali was able to escape by using her abilities.
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.
Remember also that we have evidence that powers can be inherited -- El inherited her powers from Terri Ives, for example. So it stands to reason that Henry inherited his powers from one of his parents.
And he saw the possibility for escape when he realized the extent of Eleven's untapped potential. One manipulated Eleven into plotting her own escape, which also involved getting her to remove the device in his neck. (Henry, however, did not tell Eleven the truth when he said Brenner wanted her dead.
Vecna draws power from sad and angry memories and that is the main reason he targets traumatized teens. Those teens are a well of power just there to be absorbed. This is not just a theory, Vecna himself told Eleven about the power that angry and sad memories hold.
Vecna's obsession with time appears to be linked to his hatred of humanity. He views time as a human imposition on the natural world; an attempt to impose order on nature.
What was Chrissy's Trauma? Chrissy's life appears to be free from flaws on the surface, but in reality, she is struggling terribly to conceal her depression and self-image stemming. She was struggling with these issues, which resulted in her developing an eating disorder due to her mother's verbally abusive comments.