Vecna's "right-hand man", who ultimately became his betrayer, is Kas the Bloody-Handed, a vampire whose sword, the Sword of Kas, is also an artifact.
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.
Vecna has few allies, and countless enemies. His greatest, and perhaps only true ally is the mysterious entity known as the Serpent. The famed cambion lich Acererak once served Vecna, but the current status of their relationship is unknown. Among Vecna's staunchest foes are Kas, Iuz, and the Circle of Eight.
After many years of loyal service to Vecna, Kas eventually betrayed his master. It is said that the sword itself whispered to Kas, convincing him to slay his master and usurp his power. The battle destroyed Vecna's Rotting Tower, and cost the lich his left hand and eye.
It was during the Battle of a Thousand Eyes that Kas was propositioned by Nerull to slay his master and halt the magical genocide with the reward being a godly Boon. Kas, hungry for power, obeyed Nerull and used the blade gifted to him by Vecna to make an attempt on his life.
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 episode 7, Vecna is revealed to be Henry Creel (played as a youngster by Raphael Luce), the son of Victor Creel (Robert Englund) and his wife Virginia (Tyner Rushing) shown in the 1950s flashbacks.
In the real world, he makes the victim levitate while in their trance, before snapping their bones and neck, and crushing their eyes. The murder site then becomes a small gate into the Upside Down, as part of a long term goal to apparently bleed both dimensions into one.
Eleven demonstrated the ability to overpower Vecna when she piggybacked into Max's memory and again after Vecna took her hostage in the Upside Down. Eleven was the one who first opened the gate to the Upside Down, a skill at which Vecna was less adept.
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.
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.
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.
Vecna seeks four victims in total. His first three victims — Chrissy Cunningham, Patrick McKinney, and Fred Benson — are all peers of The Party, people they know from school, which makes Vecna's terror all the more real.
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.
Vecna chooses which memories he would like to have in his subconscious, and it's only by chance that the victims he wishes to absorb are all Hawkins High students. It's odd that all of his victims are teenagers, but there could be a deeper purpose for them going into Stranger Things season 5.
“Dear Billy” is a phenomenal episode and in the moment has an air of finality. Max has escaped Vecna, so she is safe from Vecna. But you don't just beat depression once and go about the rest of your life.
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.
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).
"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."
Whenever victims become entranced or possessed by Vecna, their eyes form a milky glaze, until they meet their demise as Vecna plucks their eyeballs out. Also, when Vecna's victims hallucinate — an early indicator they've been targeted by Vecna — the people in their hallucinations have milky eyes.
In the game, Vecna seeks power through artefacts, while in the series, he enhances his powers by consuming the essence of his victims, like the other gifted children in the lab, and absorbing their powers. When he took over the Mind Flayer, he gained control over the Demogorgons and other monsters.
At his empire's height, Vecna was betrayed and destroyed by his most trusted lieutenant, a vampire called Kas the Bloody-Handed, using a magical sword that Vecna himself had crafted for him, now known as the Sword of Kas.
Vecna successfully stole Eleven's powers when controlling the Mind Flayer in Stranger Things' season 3 ending, which finally allowed him to emerge stronger than Eleven and enact his plan to take over Hawkins with the Upside Down.
But Henry Creel did not die in the hospital like Victor believes. Brenner took him and made him a confined human lab rat. Brenner wanted to use/control Henry but he couldn't. So, he decided to recreate kids like Henry, thus kicking off the program.