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.
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.
He was eventually destroyed, and his left hand and left eye were the only parts of his body to survive. Even after the character achieved godhood—being a member of the third edition's default pantheon of D&D gods (the pantheon of Oerth)—he is still described as missing both his left eye and left hand.
Season 4's main villain Vecna contains a multitude of easter eggs. To start, he has a giant, claw-like left 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 Eye and Hand of Vecna can only be destroyed by the Sword of Kas, the sentient chaotic evil sword of Vecna's former vampire general, Kas.
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.
These visions build over time, and within a space of 24 hours Vecna's victims enter one final trance. 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.
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.
Vecna prays on those dealing with trauma, which is why Max, who recently lost her brother, Billy, was the perfect victim. While she initially got away from him with help from her friends and her favorite song, she wasn't as lucky at the end of the season when Vecna captured her again.
Played by Jamie Campbell Bower, each prosthetic was glued to his skin with medical adhesive by prosthetics designer Barrie Gower and his team. For the finishing touch, Vecna was covered in "glossy slime to make him really, really glossy and wet looking," according to Gower.
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.
On the final roll of the dice in Eddie's D&D campaign, Erica manages to roll a 20. It's a critical hit, the party finally defeat Vecna and win the campaign.
What Does Vecna Want? Vecna's goal is to open as many gates to the Upside Down as he can, in order to help the Mind Flayer.
Based on the flashback in “The Hellfire Club”, he killed an estimated 21 people: 13 lab subjects, two orderlies, two guards, Ten, Alec, Dr. Ellis, and Six. A flashback during “The Massacre at Hawkins Lab” later reveals that Vecna killed even more people in Hawkins Lab than we thought.
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.
When One massacres the rest of the children in Hawkins Lab, Eleven banishes him to the Upside Down in the same way she banishes the Demogorgon from season one. One is sent hurtling through the Mind Flayer's lightning and is horribly disfigured, transforming into Vecna.
His first victim was Chrissy Cunningham. Vecna targeted Chrissy by using her depression and eating disorder against her in his visions. Chrissy was killed in Eddie Munson's trailer, and the first gate was opened on the ceiling.
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.
Vecna holds off Eleven long enough to brutally attack Max in his signature style—she levitates, her eyes go white and bleed, and he snaps her limbs. Eleven regains her strength and pushes Vecna off before he can complete the job, but the damage is already done.
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.
The 'curse' begins with the victim having severe headaches, nosebleeds and nightmares regarding any traumatic event in their past. Eventually, the cursed will have waking hallucinations, often of other people taunting them about their secrets.
Music evokes strong memories that are both good and bad, so it makes sense that it could open a doorway out of the oppressive alienation of trauma, despair, and guilt that Max found herself in.
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.
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.
"Vecna seems to be actually intrigued by Nancy being able to figure him out, to be able to get this close to the truth... I think this is more Vecna's narcissistic traits that pull him to Nancy, why he wants to give her this information."