Vecna successfully stole
Vecna attacked Hawkins with the Mind Flayer. While it seemed as though the kids had defeated the creature's physical form at Starcourt Mall, they had lost without knowing it. Vecna used the attack to steal Eleven's powers.
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).
Powers and Abilities. Harnessing psychic abilities, Vecna can detect humans in Hawkins, and just like Eleven, he can use telekinesis to kill his opponents. Unlike Eleven however, he can also use advanced telepathy and create false visions in his victims, which makes him even more dangerous.
Out of all these monsters, Vecna does have heightened intelligence along with personal motivation, which makes him the most dangerous. He also has the power to cause harm from a distance, and that makes him seem more powerful than the Mind Flayer.
The first reason that Vecna was stronger than Eleven is simple. He was older and more experienced. He was the original subject in Project MKUltra, and using his DNA, Dr. Brenner was able to make or alter the other children so that they would also have powers.
Stranger Things season 4 confirms that Jamie Campbell Bower's Vecna is pure evil and cannot be given a redemption arc that'll fit his story.
His main weakness is not any particular set of traits, but a constraint imposed by his location: the Chaotic Quest entrance is a portal rather than a staircase, and Vecna is covetous. Vecna will therefore readily warp to you, but will not teleport away to heal.
After a certain period of time, Vecna will finally kill the victim by placing them into a comatose-like state, in which they will be unable to respond to outside stimuli.
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.
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.
In simple words, Vecna needs a total of four gates to take over Hawkins and rule it. Now, you must be thinking, why is Vecna killing people if he can take over the complete Hawkins? Well, in one of the previous episodes, it was revealed that with each person Vecna kills, he creates an opening to the real world.
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.
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.
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 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.
Together, their powers combined are the exact powers of 001/Henry. Fans are now theorising that the powers of both 011 and 008, who have the combined power of 001, is what is needed to finally match and defeat Vecna. To put it simply, 011 + 008 + [with the combined power of] 001 = 020, a.k.a. Erica's critical hit.
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'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.
In 4th edition, the Open Grave book shows the leader of the cult of Vecna is a lich named Mauthereign.
If Vecna has you in a trance, listening to your favorite song can help pull you out of it, as we saw when Vecna attacked Max only for her friends to pull her out his trance using the synth-tastic stylings of Kate Bush, specifically her 1985 hit “Running Up That Hill (A Deal with God).”
Eleven can take control of the Upside Down from Vecna, but she will still need to find a way to shut down the Mind Flayer and the Upside Down. Stranger Things season 4 already showed audiences how Eleven can defeat Vecna in the Netflix series' final season.
Mike, on the other side tries to give strength to Eleven, who is at risk as she is captured by Vecna. Eleven gathers her strength and overpowers Vecna, who's just about to kill Max.
It's revealed that Vecna is actually Victor Creel's son, Henry. After realizing he has powers while feeling alone in his own home, Henry decides to attack his own family and kill them to test his power. Victor is framed for the murders at Creel House, while his son is taken away.
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).