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.
Originally appearing in the Greyhawk campaign setting, Vecna was described as a powerful wizard who became a lich. He was eventually destroyed, and his left hand and left eye were the only parts of his body to survive.
Before coming to the Hawkins National Lab, Venca had a classic villain origin story — he became obsessed with black widows, tortured his family, and brutally killed his mother and sister pinning it on his father.
The BGFX team, which consisted of about 15 to 20 people, started the process of designing Vecna by molding a full body cast of Bower, and then sculpting Vecna's prosthetics with modeling clay.
The gate eventually opening in the Creel House was either a massive coincidence or a hint that Vecna's powers include visions of the future, which would make it nearly impossible for Stranger Things' heroes to defeat him as he would see every plan coming.
While intending to get his revenge on Eleven and Brenner, Vecna also goes after victims who have dealt with traumatizing experiences in the past (reminiscent of his childhood and his time at the Hawkins lab).
There is a lot of evidence suggesting both frightening villains were behind the vanishing of Will Byers, but we have to give the win to Vecna. There is much more evidence suggesting that it was the super-powered bad guy, and even if Vecna sent a Demogorgon to kidnap Will, it would still be Vecna who is responsible.
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.
Vecna hated humanity, believing human beings had imposed an artificial order upon the natural world.
(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 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.
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.
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.
When attempting to open gates through this method, Vecna would strategically prey on teenagers with trauma, or experiencing mental health issues. He would go out of his way to psychologically torture his victims, making them experience disturbing and disorienting hallucinations, before ultimately killing them.
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.
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.
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.
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. However, the show itself doesn't confirm this.
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.
Motivated by a cruel and misanthropic philosophy, Vecna targets particularly traumatized, mentally ill or insecure individuals.
His method of killing is ritualized. Vecna started twisting bodies and removing eyes before imprisonment in the UD to fuel its own individual power. It takes the eyes to help hold the souls in darkness and because it did not wish to be observed.
A Troubled Henry Creel Moves to Hawkins. 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.
Using past traumas to break down his victims, Vecna brought Nancy to the pool where Barb was killed by the Demogorgon and revealed her mangled corpse.
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.