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.
The Demogorgon monster was created by Spectral Motion, a creature effects company from Los Angeles that has previously collaborated with Guillermo del Toro. They just had a little over two months to finish the creature, so there wasn't much room for error.
The creature originated from the parallel dimension known as the Upside Down. When Eleven, a child test subject from Hawkins National Laboratory with psychokinetic abilities, made transdimensional contact with the creature, a gate between dimensions opened at the lab.
When making Season 1, the Duffer Brothers wanted to primarily use practical horror elements to create the Demogorgon. The monster was shot partly via motion capture, with performer Mark Steger wearing a monster suit with an animatronic head on-set.
The Mind Flayer exercised supreme control of the Upside Down via a psychic link, with Demogorgons, vines, and Demobats forming a shared hive mind; for a period in 1985, possessed humans known as "the Flayed" also comprised part of the hive mind.
Next up, the viewers go on to theorise that One/Vecna is actually Eleven's dad. Yep, told you it was wild. The theory claims that Brenner wanted to create a child with stronger powers, so he waited until One was old enough to become a father.
It seems increasingly likely that Vecna created the monsters of the Upside Down as nightmare fuel for his plan to bring fear to his victims. If he did create the creatures as a way to grow his own power, Vecna's connection to all of the monsters in the Upside Down could hint at a way to make sure it falls.
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.
Barb was the best friend of Nancy Wheeler. In late 1983, Barb mysteriously vanishes, which leads Nancy and Jonathan Byers to investigate her disappearance. Eventually, it was revealed that she had been killed by the Demogorgon.
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).
The slug Will coughed up is a baby Demogorgon. Though the actors were a little hesitant to confirm this on the after-show, it seems pretty apparent that the slug Will coughed up at the end of “Stranger Things” Season 1 was Demogorgon spawn, or at least a baby Demodog.
Perhaps the only thing preventing Demogorgon from taking over the rest of the Abyss was his own dual nature. If you want to have even a chance at stopping him, you need to distract him. That is Demogorgon's weakness. Each situation he faces brings two reactions, not one.
This (literally) more cerebral monster was looking for a human host, whereas the Demogorgon that grabbed Barb was just looking for food or may have screwed up his attempt at appeasing the Mind Flayer.
In this lyrical drama, Demogorgon is the offspring of Jupiter and Thetis who eventually dethrones Jupiter.
Vecna was the original child studied by Dr. Brenner and the son of Victor Creel. He's the infamous Number One, but had his powers blocked and worked as an aide in the Hawkins National Lab.
Eleven holds the power to defeat the monster using her telekinesis powers, but she is also part of the reason it landed in Hawkins in the first place. 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.
The theory predicts that Vecna wants to regain his human form, and he will use Will to do so in Season 5. Knowing that Vecna singled out Will in Season 1, it's a reasonable theory, but it might go too far. After all, Vecna already has a human form, and he has vowed to eliminate humanity.
Purser also said that she didn't know that Barb would die at first. She explained, "there was this secrecy about Episode 7, which is the episode where [Chief Hopper] finds Barb's body.
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.
Then there were the three poor teens of Season 4. 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).
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.
Vecna, unlike the other creatures, is a human who has mutated into a monster from overexposure to the Upside Down. He's been subjected to the toxic environment for around 20 years and has become a part of it.
In a fit of rage, Henry used his restored powers to brutally murder the other test subjects and workers, and attempted to do the same to Eleven. However, Eleven stood her ground and ultimately overpowered him, sending him through a gate to the Upside Down; there, he gradually transformed into the being known as Vecna.
The Upside Down is first expected to be exposed to the real world in 1979, during Eleven's fight with One/Henry Creel in the Rainbow Room of the Hwakins Laboratory when Eleven unintentionally opened the gate while overpowering One, sending him to the dark world, where One became Vecna.