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.
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.
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'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.
In addition to telekinesis, Vecna is shown to have telepathic powers as he enters into people's thoughts and attacks them remotely from the Upside Down.
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.
"The reason why Vecna only has a single eye and hand is due to a betrayal by Kas", who used the sword against his former master. "In the third edition of Dungeons & Dragons, the Sword of Kas is a +6 unholy keen vorpal longsword that grants a +10 modifier to the Strength score of its wielder".
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.
While in this trance, Vecna will stalk his victim, chasing them through his mind, the victim's mind, and whatever strange psychic realms lie in-between. If Vecna can overwhelm their victim with fear, and keep them from resisting, their fate is almost certainly sealed.
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.
Vecna adopts psychological warfare to target his prey – which, you could argue, very much serves as an allegory for trauma, depression and mental illness, which is all too real.
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.
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 was inspired by mould
But according to Gower, they also “looked at a lot of sea life, moulds and animals to create the look, feel and consistency of his skin”. Gower says the Duffer Brothers were also “very adamant that Vecna be glossy and shiny, that he looked as moist and wet as the rest of the Upside Down”.
(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.
During the episode, Nancy and Robin learn that music is the only thing that can save people from Vecna's wrath. This bit of information proves useful as the gang is able to fend of the upside down demon and save Max using her favorite song Kate Bush's 1985 hit "Running Up That Hill (A Deal with God)."
Hawkins has suffered a lot of trauma over the years - not all of which is connected to Hawkins Lab and the Upside Down - and Vecna's curse takes advantage of that. Vecna appears to carefully choose people who feel a strong sense of guilt or shame, particularly if those emotions are repressed.
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.
While falling into the dark dimension, One/Henry is struck by lightning, disfiguring him.
Creation. 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.
Jamie Campbell Bower revealed that his Vecna suit had a "flap" in order for him to use the bathroom during shooting for "Stranger Things," but he needed some help maneuvering it.
Vecna was not always a hideous beast of the Upside Down. He was once a troubled child in the real world named Henry Creel. Henry's “problems” caused his family to move to Hawkins in 1959. However, the exact issues that led to the relocation are unclear.
"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."
Now we know that Will's connection has always been to Vecna (not the Mind Flayer). That's why he can sense that he's still alive at the end of Stranger Things Season 4. So the connection between Will and Vec — Can I call Vecna/Henry/One just “Vec”? — is one of sheer chance.
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.