He's the infamous Number One, but had his powers blocked and worked as an aide in the Hawkins National Lab. He befriended Eleven at the lab and not knowing Vecna was evil, Eleven restored his powers resulting in the massacre and his expulsion to the Upside Down.
Eleven opened the first permanent gate in 1983 after making physical contact with a Demogorgon/Vecna while in a deep psychic state.
Not only did she prove to be central to the final battle, but she was also the reason for how he became Vecna in the first place. The reveal that Number One was Vecna made it clear just how important Eleven was to Stranger Things season 4's overarching story.
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.
"I saw a means to realize my potential to transcend my human form, to become the predator I was always born to be," Vecna tells El.
"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."
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.
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.
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.
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.
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.
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).
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.
"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."
His first three victims — Chrissy Cunningham, Patrick McKinney, and Fred Benson — are all peers of The Party, people they know from school, which makes Vecna's terror all the more real.
He's the infamous Number One, but had his powers blocked and worked as an aide in the Hawkins National Lab. He befriended Eleven at the lab and not knowing Vecna was evil, Eleven restored his powers resulting in the massacre and his expulsion to the Upside Down.
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.
The balance of power seesaws back and forth with Eleven first taking the upper hand and then Vecna revealing that by banishing him to the Upside Down, El has made him more powerful than ever before. It seems Vecna has won and Max is doomed.
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.
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.
She breaks free, momentarily incapacitates Vecna, and runs with the determination to live toward her friends, dodging debris that Vecna throws in her path. Max hurls herself through the portal, awakes from the trance, and is held by Lucas (Caleb McLaughlin). "I thought we lost you," he cries. "I'm still here.
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.
Using past traumas to break down his victims, Vecna brought Nancy to the pool where Barb was killed by the Demogorgon and revealed her mangled corpse. Vecna then taunted Nancy with her own feelings of guilt - that she was responsible for Barb's death and had forgotten about her.