The rebirth of Vecna as the Arch-Lich allowed him to conquer death, and bend it to his will. He can bend the dead to his will, and should you prove worthy of his blessing, you too shall inherit a minutia of his power. His clerics can raise the dead as servants, and bend the will of death to prolong their own life.
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 his story has evolved slightly over the years, he has always been a powerful undead lich. The original appearance of the Eye and Hand of Vecna was in 1976's Eldritch Wizardry and it was said that Vecna was long-since destroyed.
Vecna is the Evil god of Undead, Necromancy, and Secrets. Specializing in dark magic and undeath, he is the go-to guy for necromancers and dark mages of all sorts.
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.
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.
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.
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.
They rush to find Vecna's body and burn it down. With this Vecna's soul turns to ash in front of Eleven. Max's dies in Lucas' arms, thus getting Vecna his fourth kill. The entire town opens up to Upside Down, with all portals dissolving.
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 Did Vecna Go After Max? As Vecna continued to go after young victims in order to open more portals, he set his sights on Max Mayfield, since she was investigating the deaths of Chrissy and another student named Fred Benson with help from her friends (including Dustin, Steve, and her boyfriend Lucas).
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.
So, Max was nearly murdered like Vecna's three previous victims. Just before Max died, Eleven rallied her strength, defeated Vecna and freed Max -- but the damage had already been done. Even though Max was free from Vecna, she died in Lucas' arms.
The character is a longtime D&D villain, one who used his abilities as a mage to turn himself into a lich (an undead magician) in order to avoid a mortal death. Despite achieving immortality, Vecna's physical body was almost completely destroyed by his follower, Kas.
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.
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.
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.
However, within the red smoke of Vecna's Mind Lair, it wasn't the song itself that saved her, but the memories it conjured up. What broke through to Max was the happy, loving memories of her friends, particularly Lucas and Eleven, and these were what truly allowed her to break free of Vecna's grasp and escape.
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.
"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."
Robin also remembered Warden Hatch's words: "music can reach parts of the brain that words can't". Putting the pieces together, the pair speculated that Victor's radio acted as a 'lifeline to reality'. They then surmised that music could protect Max from Vecna's spell.
Vecna isn't as terrifying in the bathroom. Actor Jamie Campbell Bower, who played the complicated villain in “Stranger Things 4,” revealed that his terrifying Freddy Krueger-esque full-body prosthetics included a “flap” for him to use the restroom without getting out of costume.
In the Upside Down, Eleven is being restrained by Vecna's vines while he attempts to kill Max, breaking her legs and arms before he begins to take her eyes.
The Upside Down is first expected to be exposed to the real world in 1979, during Eleven's fight with One/Henry Creel in the Rainbow Room of the Hwakins Laboratory when Eleven unintentionally opened the gate while overpowering One, sending him to the dark world, where One became Vecna.