Why do the Russians try to reopen the door to the Upside Down in Stranger Things 3? Mostly likely to try and get weapons from it. We know they captured a demogorgon, which I think came from Barbara's larvae, so they are obviously are interested in researching it and weaponising it.
The finale revolves around his pursuit of the final victim, Max. Despite the best efforts of Eleven and Lucas, Vecna succeeds in wounding Max badly enough with the ritual that she dies. Eleven manages to revive her, but the minute that Max spent dead was enough to trigger Vecna's spell and open the gate.
The ending of Stranger Things season 3 established that the Russians had access to at least one Demogorgon. However, Stranger Things season 4 confirmed that they kept multiple creatures in Kamchatka as part of their experiments. This begs the question of just how many Demogorgons the Russians had in their facility.
The Soviet Union caught wind of the Upside Down's discovery and sought to learn about its secrets; in 1985, this culminated in a new gate being opened using a bespoke machine, located in a secret base located under Hawkins's Starcourt Mall.
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).
Toward the end of Hopper's captivity, during which he befriends his prison guard (played by "Game of Thrones" actor Tom Wlaschiha), it's revealed that the Russians have been keeping a Demogorgon hostage and feeding it unruly prisoners.
Looking for an edge beyond nuclear proliferation, the Soviets sneaked into Hawkins, where they deployed a giant laser beam to crack open the same gate to the Upside Down that Eleven (Millie Bobby Brown) and her friends labored so hard to seal up.
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.
The Soviets, having opened a stable gate to the Upside Down with experimental technology on June 28, 1985, apparently used the gate to extract various 'Demogorgon' specimens; somehow, the Soviets also captured a mass of silver-black particles (a portion of the Mind Flayer).
Vecna was not always a hideous beast of the Upside Down. He was once a troubled child in the real world named Henry Creel. Henry's “problems” caused his family to move to Hawkins in 1959. However, the exact issues that led to the relocation are unclear.
A separate piece of the Mind Flayer was captured by Soviets and stored at a prison camp in Kamchatka; it was presumably extracted via the Soviets' gate to the Upside Down at their base under Starcourt Mall.
Grigori is one of the two secondary antagonists (alongside Billy Hargrove) of the third season of Stranger Things.
Robin laughs at the absurdity of the situation, reminiscing about how she shared a history class and was obsessed with Steve, and he was not even aware that she existed. The Russians arrive with a doctor, Zharkov, who injects them with a truth serum.
But after Joyce and Murray break in to break Hopper out, the trio plus Enzo/Dmitri learn that the truth is far more sinister: the Russians have been breeding and potentially modifying Demogorgons in the depths of the prison, and have essentially bottled what Joyce calls “particles" from the Upside Down.
The most obvious solution is that the Russian's captured the Demogorgon in Hawkins and shipped it back to Russia. After all, Dustin and Erica did see a Demogorgon-sized cage below Starcourt Mall.
Demogorgon is first mentioned in the commentary on Statius's Thebaid often attributed in manuscripts to a Lactantius Placidus, (c. 350–400 AD). The Lactantius Placidus commentary became the most common medieval commentary on the poem by Statius and is transmitted in most early editions up to 1600.
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).
"Vecna seems to be actually intrigued by Nancy being able to figure him out, to be able to get this close to the truth... I think this is more Vecna's narcissistic traits that pull him to Nancy, why he wants to give her this information."
In his twisted mind, Vecna believes that he his helping the world by wiping out humans, as a "predator, but for good." When Vecna asks for El to join him, it's because he also sees her as a predator, "better" than humans. He offers her his path as a sort of freedom, as they reshaped the world in their image.
It was Hopper's supposed demise that pushed Joyce to sell her house and leave Hawkins, taking her sons and Eleven with her, in the hopes of starting a new life. However, despite this, Joyce looked reluctant to leave Hawkins as she saw how deeply affected the kids were in saying good-bye to their friends.
There are multiple Demogorgons, and they can be trained to become stronger and even more murderous, which viewers see the Russians doing in season 4.
A Russian agent, portrayed by Andre Pushkin, was a minor character in the first season of Stranger Things. In 1983, he discussed with an unknown individual(s) about Russian spies on American soil.
However, it was unable to resist Eleven's power of biokinesis, which she uses to dissolve and kill the Demogorgon. Telekinesis: The Demogorgon appeared to have had some form of telekinetic power.
Re-watching the episode where Will Byers goes missing, there is a lot of evidence suggesting that it was Vecna who abducted Will and trapped him in the upside down. Many Stranger Things fans who re-watched the scene pointed out that they could hear the chimes of the grandfather clock.