Vecna had his own strength, combined with the strength of everyone he had killed and absorbed. That means that he wielded his own power, combined with the power of all Brenner's test subjects and the various monsters he had dominated in 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.
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.
While both are powerful beings from the Upside Down capable of taking over Stranger Things' lesser monsters like Demogorgons, Demodogs, and Demobats, the heightened intelligence and personal motivation Vecna possesses likely make him more dangerous.
The Mind Flayer is more powerful than Vecna. In Stranger Things, who's father do you think is worst Neil Hangrove or Lonnie Byers?
While the source of Vecna's powers is never explained, there are some Stranger Things hints leading to the theory that Hawkins had a big part in awakening them. Vecna is revealed in season 4 to be the base cause behind all of Hawkins' unfortunate happenings for the past three years.
Vecna is the Source of Eleven's Powers in Stranger Things
However, things turn ugly when Brenner kidnaps Henry to control his powers. Since Henry couldn't be tamed, Brenner used his blood to create other kids with similar powers. The doctor then suppressed Henry's abilities by injecting a device into his neck.
Despite his Pure Evil status, Vecna's actor, Jamie Campbell Bower, stated that he sympathizes with his character, as he believes he snapped due to feeling disenfranchised by society and that he did have some genuine care for Eleven until she rejected him.
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.
The simple answer is that Vecna controlled the Upside Down's hive mind. So, here's an explanation of why the Upside Down has a hive mind and how Vecna came to control it. Season 1 of Stranger Things felt very isolated. The gate, the Demogorgon and Will's kidnapping all felt like a singular anomaly.
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 Hand
This is a reference to the D&D character Vecna, for whom this Vecna is named; the D&D Vecna has a monstrous hand infused with dark magic. The design of Vecna's hand is also a nod to Freddy Krueger, the Nightmare on Elm Street villain who had a huge influence on this season in general.
From the beginning, Vecna was a long-vanquished lich—a wizard who used dark forbidden magics to keep themselves effectively immortal but in a perpetual state of undead decay—and while DMs were free to use their imagination and flesh him out into a proper villain, his only official existence in the old D&D books was as ...
In a fit of rage, Henry used his restored powers to brutally murder the other test subjects and workers, and attempted to do the same to Eleven. However, Eleven stood her ground and ultimately overpowered him, sending him through a gate to the Upside Down; there, he gradually transformed into the being known as Vecna.
What is the significance of 4 for Vecna? The new season shows that the four chimes of the grandfather clock symbolize the four victims Vecna needs to claim in order to enter our dimension and take over humanity.
Combined with the fact that Dr. Brenner (Matthew Modine) states in the Stranger Things episode "Papa" that Vecna "consumes" everything about his victims, Vecna targets traumatized victims because it builds for him a well of sad and angry memories to draw from. With each victim, he can become gradually more powerful.
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.
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.
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.
Curiously, he also seems obsessed with time, projecting strange visions into his victims' minds—including of the grandfather clock from his home in the Creel House. Vecna's obsession with time appears to be linked to his hatred of humanity.
Vecna's curse refers to Vecna's predominant method of murder and torture. Using his supernatural psychokinetic abilities, Vecna can probe and influence the minds of others. Motivated by a cruel and misanthropic philosophy, Vecna targets particularly traumatized, mentally ill or insecure individuals.
Vecna hated humanity, believing human beings had imposed an artificial order upon the natural world.
Yeah, Bower says Brown was crying real tears during that scene. “She was terrified and freaked out. And Caleb [Heymann], our DP, was very smart and saw that she was having this reaction in the beginning of rehearsal and decided to shoot her first. It's so pure and beautiful,” Bower said.
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. He offers her his path as a sort of freedom, as they reshaped the world in their image.
Patrick was targeted due to his father's abuse of him. Finally, Vecna targets Max for her trauma from witnessing Billy's death and uses Barb's (Shannon Purser) death as a way to use Nancy.