"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 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.
Why does Vecna have one claw? He uses his left hand for his powers, and in actual DnD, Vecna's left hand is cut off and becomes a powerful magic item.
He views time as a human imposition on the natural world; an attempt to impose order on nature. Given this is the case, his use of clocks to haunt his victims could potentially be a twisted sense of irony, turning the human desire for order against them.
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.
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.
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.
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.
It seems that the reason why Vecna wasn't killing for the seven years after the Hawkins Lab massacre was because he was waiting to become more powerful as the Mind Flayer and Demogorgons took more victims, as he was likely weakened after being thrown into the dimension.
"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".
"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."
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).
After inflicting protracted suffering, Vecna finally executes the victim, using his powers to make them levitate before snapping their limbs and gouging out their eyes.
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.
From the beginning, Vecna was a long-vanquished lich—a wizard who used dark forbidden magics to keep themselves effectively immortal but in a perpetual state of undead decay—and while DMs were free to use their imagination and flesh him out into a proper villain, his only official existence in the old D&D books was as ...
What Does the Grandfather Clock in "Stranger Things" Mean? 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.
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.
If he couldn't physically leave the Upside Down, Vecna needed to use the Mind Flayer to attack Hawkins. He also needed to figure out how strong Eleven's power had gotten since their last face-off. Through Demogorgons and The Mind Flayer, Vecna began to slowly break down the barriers between the two worlds.
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.
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 finale, Will reveals to Mike that he has retained some kind of link to the Mind Flayer, and that he can "feel" that Vecna is still alive.
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.
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.
We know Eleven was born in 1971, which means her mom got pregnant with her during the tests, sometime in 1970. A 1959 flashback in the mid-season 4 finale reveals a 12-year-old One (Jamie Campbell Bower). That puts One, a.k.a. Vecna, as 24 when Eleven was born.
He didn't name himself Vecna, the kids did. They named him after a DnD character who's a wizard, since Vecna's curse sounds like something a dark wizard would do.