One/Vecna had his powers blocked and was working as an aide at Hawkins National Lab where he befriended Eleven. Not knowing that One was evil, Eleven restored his powers which he used to gruesomely massacre the entire lab.
Vecna is a dark mirror of Eleven herself, sharing all her powers, and potentially others as well. A series of flashbacks revealed Vecna was originally Henry Creel, a boy who grew up possessing strange psychic powers.
This caused her to "re-experience" her memories from 1979, including her interactions with a mysterious orderly. Eventually, Eleven remembered that the orderly was actually Henry Creel, and that he was responsible for the September 8 massacre of her fellow test subjects.
What does Vecna want with El? 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.
"I walked up to her [as Vecna] and she burst into tears. She wouldn't look at me and she was visibly just disgusted by the whole thing... After she cried and I made it obvious that she knew it was me, one of the things she said was that, 'I knew it was you when I could smell cigarettes,' because I'm a smoker."
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.
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).
In the series, Eleven is shown moments from her past at Hawkins Laboratory, right up until she manages to banish Vecna into the Upside Down.
Eleven is able to blast Vecna away from Max at first, but once the surprise wears off, he defeats El, restrains both of the girls, and takes them to his red-tinged lair (which reminds me of Sherlock's mind palace, so I'm going to call it a mind palace).
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 successfully stole Eleven's powers when controlling the Mind Flayer in Stranger Things' season 3 ending, which finally allowed him to emerge stronger than Eleven and enact his plan to take over Hawkins with the Upside Down.
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 successfully stole Eleven's powers when controlling the Mind Flayer in Stranger Things' season 3 ending, which finally allowed him to emerge stronger than Eleven and enact his plan to take over Hawkins with the Upside Down.
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.
The fan-favorite character ended the season in a coma with her bones broken and eyes blinded at the hands of Vecna. Max technically died for one whole minute before Eleven revived her friend, but Eleven's powers weren't enough to bring Max fully back to life.
Sharing more about Eleven's real father Andrew Rich could be a great way to wrap up the story.
Vecna, who absorbs beings' consciousnesses and resultantly feeds off their power, knew he had stumbled upon exactly what he needed to exact his revenge on the world: I saw so many things, then one day I found the most extraordinary thing of all. Something that would change everything.
As Sink put it: “You kind of learn in the season that he targets people that are in a real [bad] place, and Max is obviously one of those people. Just with everything with Billy, so she was kind of like the perfect target.
"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."
At his empire's height, Vecna was betrayed and destroyed by his most trusted lieutenant, a vampire called Kas the Bloody-Handed, using a magical sword that Vecna himself had crafted for him, now known as the Sword of Kas.
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.
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.
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.
Refusing to allow Max to die after their battle with Vecna, Eleven pulls out the stops to save her life. As Lucas begs Max to stay alive, cradling her in his arms, Eleven uses just enough power to get Max's heart beating again.
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.