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.
That means Vecna, Henry Creel, One, and the Mind Flayer are all the same creature, and they have been responsible for every attack on Hawkins.
In fact, he is Number One, Brenner's very first experimental subject for enhancing telekinetic powers. At this point, all the floating backstories we've learned throughout the season all come together. Number One is actually Henry Creel, Victor Creel's son and assumed victim of 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 hawkins crew calls vecna/one/henry "vecna", because he remind them of vecna from d&d.
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.
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.
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.
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.
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.
She “kills” him, flinging him into the Upside Down realm where he burns and turns into Vecna. We find out in Volume 2 that he is the first person to ever enter this uncharted realm and he uses it to his advantage. This also perfectly explains why Vecna's Upside Down spot is the Creel House.
He psychically murders his mother at dinner, killing her in what will become Vecna's signature style. Victor, who doesn't realize his son is behind this, tries to rush him and his sister from the house, but Henry stops them, killing his sister. Before he can kill his father, however, he passes out from the strain.
According to Henry Creel, he was brought to Dr. Brenner when his mother caught on to his mysterious psychic powers.
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.
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).
Terry becomes pregnant over the course of her participation in MKUltra by her adoring boyfriend, Andrew, who gets kicked out of school because of protesting. But Terry doesn't find out that she's carrying a child until after Andrew is drafted into the army.
This is because his trusted accomplice, Kas the Bloody-Handed, betrayed him and used the sword Vecna created himself to chop off his left hand and remove his left eye. This ultimately destroyed Vecna. Though they're clouded with cataracts, Vecna in Stranger Things has both of his eyes.
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.
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.
His first victim was Chrissy Cunningham. Vecna targeted Chrissy by using her depression and eating disorder against her in his visions. Chrissy was killed in Eddie Munson's trailer, and the first gate was opened on the ceiling.
We know Eleven was born in 1971, which means her mom got pregnant with her during the tests, sometime in 1970. A 1959 flashback in the mid-season 4 finale reveals a 12-year-old One (Jamie Campbell Bower). That puts One, a.k.a. Vecna, as 24 when Eleven was born.
Joe Manganiello's character Arkhan fought against Vecna with the adventuring party Vox Machina in the web series Critical Role. He then stole the Hand of Vecna and replaced his own hand with the artifact.
"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."
Following Eleven's resistance, Vecna sought after Eleven's 'power', which he identified as "a means to open [his] own doors" (gates).
If the eyes and the right hand weren't enough to convince you already, it's confirmed that Vecna was once human – and he has a very strong connection to Eleven and the Hawkins Lab.