During the match,
The history of the Upside Down remains mysterious. Exactly how and why it came into existence is unclear. Dustin theorised the Upside Down had existed for thousands, if not millions of years.
Millie Bobby Brown spoke about this in an interview with Variety in 2022. When the actress was asked if the Duffer Brothers explained that Eleven creates the Upside Down when she interacts with 001, she said, "They did tell me. She does. She opens the rift."
In the debut episode of Stranger Things season 1, Will was taken into the Upside Down by the Demogorgon. The details surrounding his abduction were never clarified in the Netflix series but the Stranger Things comic miniseries published by Dark Horse Comics filled in many blanks.
Eleven makes psychic contact with the Demogorgon, and in her terror, she accidentally rips open a hole in the fabric of reality. A gate forms in the depths of Hawkins Lab that connects our world to the Upside Down, which allows the Demogorgon to travel through.
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 Eleven's true origin story, she faces off against One after the latter threatens to destroy humanity. She is able to rip open a door to the Upside Down and sends One through, where he eventually becomes Vecna.
It uses Will because of the connection Will has to Eleven, who would be able to open further gateways to parallel universes so the Mind Flayer would be able to continue this cycle nearly indefinitely. As the Redditor continues, it becomes clear why Will could be the Demogorgon.
Vecna was the one who originally kidnapped Will in season 1.
In season 4, Peter Ballard notes that Kali was no longer at the lab in 1979. Given Kali's powers, it seems pretty self-explanatory as to how she was able to escape the lab. She either made herself invisible or was able to escape after manipulating the minds and visions of the guards.
Brenner's words make it seem like Vecna subjugated the entire Upside Down and, thereby, created the realm's hive mind. In other words, the Upside Down's hive mind is actually Vecna's mind because everything that he dominates becomes a part of him.
Vecna taps into the trauma in a human brain to cause pain while the Mind Flayer doesn't have a human brain, so Vecna's powers become useless against it. However, if Vecna can tap into The Upside Down's hive mind, he may be able to take down the Mind Flayer.
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.
However, in the finale of season 4, "Chapter Nine: The Piggyback," Vecna describes the Upside Down as "a realm unspoiled by mankind." It is only after Vecna's arrival in the Upside Down that it presumably is polluted by him.
The Russians then discovered that location was vital and that the gate was once opened before in Hawkins, IN, thus building a secret base beneath Starcourt Mall. Alexei reveals that the gate is opening. (And, as we see, allowing the Mind Flayer to invade Hawkins.)
Using past traumas to break down his victims, Vecna brought Nancy to the pool where Barb was killed by the Demogorgon and revealed her mangled corpse. Vecna then taunted Nancy with her own feelings of guilt - that she was responsible for Barb's death and had forgotten about her.
Will Became Connected to Vecna During His Kidnapping in the Upside Down. The assumption has always been that Will survived in the Upside Down on sheer dumb luck before being trapped in those creepy hive mind vines.
Presumably, Vecna chose to conceal his true identity as he spoke through Billy, explaining some of the strange pronouns used during the conversation.
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). But wait, there's more.
Trivia. The Demogorgon Dungeons & Dragons figure, used by Eleven to symbolize the Monster. The Demogorgon received its nickname from Eleven using the Demogorgon game piece from a Dungeons & Dragons set to show that Will was hiding from the creature in her vision of The Upside Down.
The Demogorgon Tried (& Failed) To Use Barb
Barb's death implied that whatever the slug does didn't take, so the Mind Flayer reached out to Will instead. It could be that the Mind Flayer needed a younger host, or Barb may have simply died while the Demogorgon was attempting to infect her.
Then, Vecna cursed and killed Patrick, Lucas' basketball teammate. Patrick's visions were not shown. In killing Patrick, Vecna opened a huge gate at the bottom of Lover's Lake.
Sharing more about Eleven's real father Andrew Rich could be a great way to wrap up the story.
How did the Russians know about the Upside Down? Was their discovery accidental? Based on the show's timeline, the Russian scientists had a version of their machine built by about seven months after Eleven accidentally opened the Gate inside Hawkins National Lab (HNL).