He is from Hawkins laboratories who seeks to destroy the Human race after tricking Eleven into helping him escape. He is also the Mind Flayer, as revealed in season 4.
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. Vecna was the original child studied by Dr. Brenner and the son of Victor Creel.
Like Eleven (Millie Bobby Brown), Vecna — then still in human form as young Henry — was a test subject at Hawkins National Laboratory; in fact, he was the very first test subject, judging from his "001" wrist tattoo.
Netflix Vecna has plans for world domination. Essentially, Vecna hates humanity, and wants to take over their world. He had hoped that Eleven would help him achieve this goal. But when he realised she wouldn't, Vecna instead used her.
The Vecna/Mind-Flayer retcon also affects the end of Stranger Things season 3, in which Eleven was stripped of her powers. Few people would know more about Eleven's powers than Vecna; he had been an orderly at Hawkins Lab, and he'd surely have taken the chance to study Dr.
He reveals what looks to be a tracking device implanted in his neck. As a thanks, Eleven uses her powers to remove the chip, which leads the orderly to reveal his true identity as 001. 001 was born Henry Creel, the son of Victor Creel (played by A Nightmare on Elm Street's Robert Englund).
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).
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.
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.
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.
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.
Orcus, the demon prince of undeath, taught Vecna a ritual that would allow him to live on as a lich. Beyond death, he became the first and greatest of all liches.
There's been debate as to whether or not this scene implies that Eleven created the Upside Down or merely opened the first gate to it. But according to Millie Bobby Brown, the Upside Down existed before El ever meddled with it. “I don't think that she created the Upside Down,” Brown told Variety in June.
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. Brenner often lied, but his actions towards Eleven were about control and unlocking her powers, not killing her.
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.
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.
Vecna did not just kill his victims he haunted them with their traumatic past before completing the deed but his 1986 victims were the only ones who were not tortured before their death. The main reason could be that Vecna was powerless during that period and killing them was the only way he could escape.
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.
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.
He's been subjected to the toxic environment for around 20 years and has become a part of it. His silhouette incorporates sharp shapes so that, even as a slow-moving character, his toothy appendages and jagged edges make him too dangerous to touch.
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.
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).
If Vecna can overwhelm their victim with fear, and keep them from resisting, their fate is almost certainly sealed. In the real world, he makes the victim levitate while in their trance, before snapping their bones and neck, and crushing their eyes.
As Vecna, he uses these mental abilities as a way to psychologically weaken and lure in his victims – much in a way that Pennywise, Freddie Kruger, and Pinhead do. (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.
"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."