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.
Once they pop the cassette in Max's Walkman, a window opens to the real world, allowing Max to break the trance and escape Vecna's clutches.
Steve, Lucas, and Dustin figure out that the way to save people from Vecna's curse is to play their favorite song. The song that saved Max was her favorite song, “Running Up That Hill” by Kate Bush. Knowing Stranger Things, this wasn't just a random choice.
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.
Vecna's curse refers to Vecna's predominant method of murder and torture. Using his supernatural psychokinetic abilities, Vecna can probe and influence the minds of others. Motivated by a cruel and misanthropic philosophy, Vecna targets particularly traumatized, mentally ill or insecure individuals.
As Vecna, he uses these mental abilities as a way to psychologically weaken and lure in his victims – much in a way that Pennywise, Freddie Kruger, and Pinhead do. (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.
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).
Mike, on the other side tries to give strength to Eleven, who is at risk as she is captured by Vecna. Eleven gathers her strength and overpowers Vecna, who's just about to kill Max.
If Vecna has you in a trance, listening to your favorite song can help pull you out of it, as we saw when Vecna attacked Max only for her friends to pull her out his trance using the synth-tastic stylings of Kate Bush, specifically her 1985 hit “Running Up That Hill (A Deal with God).”
When beings from the Upside Down like Vecna or the Mind Flayer take control over Stranger Things' characters, music and happy memories have the power to snap them out of it and bring them back to reality in a similar manner due to these psychological connections.
The Eye and Hand of Vecna can only be destroyed by the Sword of Kas, the sentient chaotic evil sword of Vecna's former vampire general, Kas.
What Does Vecna Want? Vecna's goal is to open as many gates to the Upside Down as he can, in order to help the Mind Flayer.
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.
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.
Music evokes strong memories that are both good and bad, so it makes sense that it could open a doorway out of the oppressive alienation of trauma, despair, and guilt that Max found herself in.
She is suffering, but instead of giving in she chooses to fight and, more importantly, to keep fighting. “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.
Vecna's curse causes people to bleed before they are killed, which is probably linked to the Upside Down being partially toxic to humans or at least warping their bodies due to exposure.
While the season 4 marks Vecna's first murder streak in Hawkins, it turns out the monster first emerged in the '50s. Super-sleuth Nancy finds out that the monster previously targeted the Creel family, led by patriarch Victor Creel.
Vecna's first big credited kill was Chrissy Cunningham (Grace Van Dien). He followed that murder with canonically terrible driver Fred Benson (Logan Riley Bruner) and basketball star Patrick McKinney (Myles Truitt).
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.
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.
Vecna's obsession with time appears to be linked to his hatred of humanity. He views time as a human imposition on the natural world; an attempt to impose order on nature.
Vecna was the one who originally kidnapped Will in season 1.