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.
The trio gets upstairs and find Vecna strung up on his vines, unconscious while he deals with Eleven in the mind palace. They torch him with Molotov cocktails, and when he wakes up, Nancy shoots him with a sawed-off shotgun.
Together, their powers combined are the exact powers of 001/Henry. Fans are now theorising that the powers of both 011 and 008, who have the combined power of 001, is what is needed to finally match and defeat Vecna. To put it simply, 011 + 008 + [with the combined power of] 001 = 020, a.k.a. Erica's critical hit.
Meanwhile, in Russia, Murray is able to torch the demogorgons and demo dogs there. This weakens the Upside Down at the perfect moment. Nancy, Steve, and Robin torch and shoot Vecna's physical body to smithereens.
The Mind Flayer is more powerful than Vecna.
His main weakness is not any particular set of traits, but a constraint imposed by his location: the Chaotic Quest entrance is a portal rather than a staircase, and Vecna is covetous. Vecna will therefore readily warp to you, but will not teleport away to heal.
Max briefly dies and the portal begins to open to The Upside Down, but Eleven throws a monkey wrench in Vecna's plan by bringing Max back to life.
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).
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.
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.
Based on the flashback in “The Hellfire Club”, he killed an estimated 21 people: 13 lab subjects, two orderlies, two guards, Ten, Alec, Dr. Ellis, and Six. A flashback during “The Massacre at Hawkins Lab” later reveals that Vecna killed even more people in Hawkins Lab than we thought.
The second important note here is that he's still not easy to defeat. Some of his most notable features are that he has 32d8 + 128 hitpoints, five legendary resistances per day and can cast a counterspell as a reaction once per turn. It's probably also worth mentioning that when you defeat Vecna, he never really dies.
Realizing he had tremendous psychic power, he haunted his family with visions before ultimately killing most of them. His father was framed for the murders and locked away in a mental hospital as a disturbed serial killer. Henry then found himself in the care of Brenner, who decided he wanted more kids like the boy.
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.
Connecting to the Mind Flayer's hive mind allowed Henry Creel to become the version of Vecna that audiences see in Stranger Things season 4, controlling the Demobats, Demodogs, and Demogorgons across the Upside Down and the real world.
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.
Vecna draws power from sad and angry memories and that is the main reason he targets traumatized teens. Those teens are a well of power just there to be absorbed. This is not just a theory, Vecna himself told Eleven about the power that angry and sad memories hold.
Patrick was targeted due to his father's abuse of him. Finally, Vecna targets Max for her trauma from witnessing Billy's death and uses Barb's (Shannon Purser) death as a way to use Nancy.
After learning how Victor Creel was able to evade death back in 1959, Robin and Nancy deduce that playing the victim's favorite song can help them escape the trance that Vecna places them in, with music and happy memories being the only way to keep Vecna's fatal curse at bay.
Vecna was trained in the art of sorcery by his mother Mazzel, who was eventually executed by the government of Fleeth for practicing witchcraft.
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.
Through the window, Max saw her real body levitating above the cemetery, her friends panicking on the ground below. Bracing herself, Max pulled at a vine in Vecna's neck, causing him to writhe in pain and accidentally release her from the vines' grasp.
The Duffer brothers have also confirmed that at the end of Season 4, Max is alive yet "brain dead, blind, and all of her bones are broken." The big mystery is where the redhead is mentally.
One of the most confusing parts of season 4 of Stranger Things is Eleven being able to revive Max. As far as fans know, Eleven hasn't been able to do this before. It's not clear how she gained the power to revive someone or if she would be able to do it again in the future.