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Vecna's powers allow him to form psychic connections with people in reality, especially teenagers dealing with trauma and mental health issues. Vecna develops these connections with young and vulnerable teens so that he can get out of the Upside Down and gain access to the real world by opening four gates or portals.
There, Vecna taunts them, rejoicing in their heartache and sorrow, before killing them. Vecna's victims die in an agonizing way, lifted high into the air. Their every joint is twisted out of shape by Vecna's telekinesis, making for a horrific spectacle.
Vecna puts a curse on his victims by finding people who are dealing with tragedies and negative feelings like low self-esteem. When he has begun his process, his victims feel sick, their noses bleed, they have headaches, and they see things that aren't really there, such as a grandfather clock.
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.
In the real world, he makes the victim levitate while in their trance, before snapping their bones and neck, and crushing their eyes. The murder site then becomes a small gate into the Upside Down, as part of a long term goal to apparently bleed both dimensions into one.
In the game, Vecna seeks power through artefacts, while in the series, he enhances his powers by consuming the essence of his victims, like the other gifted children in the lab, and absorbing their powers. When he took over the Mind Flayer, he gained control over the Demogorgons and other monsters.
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 really is a creature that has mutated in the last however many couple of decades during his time in the Upside Down," prosthetics designer Barrie Gower said. "This is where he has become more overtaken by the Upside Down world and become pretty much possessed and mutated."
Vecna lives in the Upside Down and preys on people's past traumas and guilt. The monster curses its victims, making them relive their trauma in progressively more gruesome ways until it violently kills them.
And while his usual modus operandi is to embody his victims' negative self-talk (another very real psychological phenomenon), this time he is telling Nancy his plans and then releasing her so she can report back to everybody—Eleven included—that the end is nigh, with the goal of discouraging and disempowering them.
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.
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 nabs Eleven and Max. Nancy, Robin and Steve, who were tasked to destroy Vecna's body when his soul is inside Mind Lair, also get trapped by his tentacles.
The theory predicts that Vecna wants to regain his human form, and he will use Will to do so in Season 5. Knowing that Vecna singled out Will in Season 1, it's a reasonable theory, but it might go too far. After all, Vecna already has a human form, and he has vowed to eliminate humanity.
He starts to resemble… Vecna. The episode then cuts to Vecna in 1980s Upside Down, zooming in on his arm, where a tentacle slides away to uncover 001 tattooed on his wrist. So Vecna is 001, who is also Victor Creel's son.
This ultimately destroyed Vecna. Though they're clouded with cataracts, Vecna in Stranger Things has both of his eyes.
The Hand and Eye of Vecna were all that remained of the arch-lich Vecna after his betrayal at the hands of his trusted vampire lieutanant, Kas the Bloody-Handed. Using a sword crafted by Vecna himself, Kas manage to cut off Vecna's left hand and left eye before in turn being destroyed.
At the same time, Nancy, Steve and Robin reach Vecna's body in the Upside Down and throw multiple Molotov cocktails at him before Nancy shoots him, sending his blazing body flying out the window of the Upside Down's Creel House.
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.
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.
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.
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.
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.