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.
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.
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.
But new villain Vecna is a reflection of something very real: post-traumatic stress disorder, depression and grief.
Vecna also eventually targets Max (Sadie Sink) and Nancy (Natalia Dyer) for their own traumatic pasts regarding the deaths of people important to them.
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.
Once the monster catches them, as he inevitably does, Vecna kills them from the inside out by taking their soul while their body in the real world is broken in horrific angles and their eyes are gouged out.
Jennifer Chaiken, LMFT
And that's why I think having Vecna as a representation of [PTSD and] depression can be really helpful for those who do struggle with [these issues] as a way of externalizing [and depersonalizing] them.”
(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.
In other words, he's the big, scary monster wreaking havoc on Hawkins. 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.
Why does Vecna mutilate his victims? Dr. Brenner (Matthew Modine) clarifies that Vecna "consumes" his victims, as he isn't interested in simply killing them - meaning Vecna takes everything about and from a person.
One of the most important sequences in this episode featured Vecna showing Nancy the real circumstances of the “eyeball murders” in the 1950s, with flashbacks indicating that young Henry Creel murdered his family when his mother tried to get him psychological help and he intentionally framed his father Victor for the ...
In the visions of her trauma induced by Vecna, Chrissy's mother appeared both mentally and emotionally abusive towards her daughter - and it's implied her comments about Chrissy's figure caused her daughter to develop her eating disorder.
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.
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.
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.
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.
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.
Since he's been the puppet master behind this entire series, it's not insane to at least partially blame him for the deaths caused by various Demogorgons, Upside Down bats, and the Mind Flayer. Basically, if you only count direct deaths that have stood the test of time, Vecna is up to 40.
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.
He is terrifying because he has access to secrets and the guilt that accompanies those secrets. Vecna's use of his victims' guilty conscious to terrorize his prey is an obvious tool in his arsenal.
After learning how Victor Creel was able to evade death back in 1959, Robin and Nancy deduce that playing the victim's favorite song can help them escape the trance that Vecna places them in, with music and happy memories being the only way to keep Vecna's fatal curse at bay.
In the D&D realm, a defining feature of Vecna is that he's missing his left hand and left eye. This is because his trusted accomplice, Kas the Bloody-Handed, betrayed him and used the sword Vecna created himself to chop off his left hand and remove his left eye. This ultimately destroyed Vecna.
There are still a few details we don't know, but it's clear that Brenner secretly kept Henry around as an orderly and the chip in his neck was some kind of power suppressant. Without it, Henry is able to unleash his powers on those in the lab, slaughtering nearly everyone, including the children.
When he got older, Henry's powers grew intensely and his intentions weren't always harmless. This led Dr. Brenner to implant a chip in his neck that suppressed his powers. In the volume 1 finale, it's revealed that he manipulated Eleven — the only subject more powerful than he is — to remove it for him.