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. Though they're clouded with cataracts, Vecna in Stranger Things has both of his eyes. For now, at least.
Once the left eye of the arch-lich, Vecna, the Eye, and its sister artifact, the Hand of Vecna, were all that remained of the lich-king after his battle with his renegade lieutenant, Kas the Bloody-Handed. Wherever the Eye appeared after that, woe followed.
Arkhan had taken and attached the Hand of Vecna in the hopes to use such a powerful magic item to free his deity, the Scaled Tyrant, from her prison in Avernus (first of the Nine Hells). With the Hand in his possession, he Teleported to his keep in Avernus.
When they presented Pelor with the eye of Vecna, he offered to destroy it. Once it was destroyed, Vex asked Pelor about the goddess Ioun, as they needed information and power to defeat Vecna.
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. Vecna's holy symbol is an eye in the palm of a left hand.
"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."
Next up, the viewers go on to theorise that One/Vecna is actually Eleven's dad. Yep, told you it was wild. The theory claims that Brenner wanted to create a child with stronger powers, so he waited until One was old enough to become a father.
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.
Nancy, Steve and Robin get freed too. 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.
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.
That's right: Vecna claims his second victim, and it is … Fred Benson, Nancy's right hand at the school paper. Nancy and Fred head over to the trailer park to get the scoop on Chrissy's death, but as soon as they pull in, Fred starts to have the same type of hallucinations Chrissy did.
At the height of his empire Vecna was betrayed by Kas. Kas managed to destroy the Archlich before his own death, with the Sword of Kas, a lethal blade of the Undying King's own make, leaving only Vecna's left hand and left eye behind. Vecna eventually returned and by 581, had risen to godhood.
One/Vecna had his powers blocked and was working as an aide at Hawkins National Lab where he befriended Eleven. Not knowing that One was evil, Eleven restored his powers which he used to gruesomely massacre the entire lab. Eleven banished One/Vecna to the Upside Down where he's been attacking the Hawkins teens from.
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.
The final scene of Volume 1 shows a shot of Vecna, and a close up of the 001 tattoo on his arm, confirming that Henry Creel, a.k.a. Number One, became Vecna.
Vecna had his own strength, combined with the strength of everyone he had killed and absorbed. That means that he wielded his own power, combined with the power of all Brenner's test subjects and the various monsters he had dominated in the Upside Down.
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).
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.
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.
For all his power, Vecna couldn't escape his own mortality. He began to fear death and take steps to prevent his end from ever coming about.
The latter tactic is exactly how Billy very briefly escaped the Mind Flayer's influence before sacrificing himself in Stranger Things season 3. Vecna's weakness against the power of music will become an important point in Stranger Things season 4, volume 2 when the characters have to save Nancy from Vecna's curse.
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 is a dark mirror of Eleven herself, sharing all her powers, and potentially others as well. A series of flashbacks revealed Vecna was originally Henry Creel, a boy who grew up possessing strange psychic powers.
Vecna attacked Hawkins with the Mind Flayer. While it seemed as though the kids had defeated the creature's physical form at Starcourt Mall, they had lost without knowing it. Vecna used the attack to steal Eleven's powers.
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.