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.
Blood Detection: The Demogorgon was strongly attracted to blood and could sense even small amounts from different dimensions.
If blood isn't what attracts the Demogorgon, then it might take those who, unfortunately, are close to the gate. Another fan theory suggests the Demogorgon targets the living creatures near the gates to the Upside Down, as the movement and sound make it aware of their presence.
Most victims, like Barb, attracted the Demogorgon because they were bleeding prior to the attack. Will, on the other hand, was stalked along the road and snatched in his shed seemingly unprovoked. It implies that he was specifically targeted as opposed to hunted opportunistically like Barb and the others.
Via this psychic contact, a tear within the fabric of space-time was created; a non-temporary gate to the Upside Down. Through the Gate, the Demogorgon traveled into Hawkins. There, it abducted Will Byers and killed Barbara Holland.
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.
It was revealed in Stranger Things season 4 that Vecna has been pulling the strings behind the Upside-Down's incursion throughout the show's events, so he may have intentionally kept Will alive in the Upside Down.
Why did the Demogorgon take Barb but spare Will? According to one Stranger Things fan theory, the creature that captured Will was being controlled by future baddie the Mind Flayer.
Eventually, the Demogorgon found them, breaking into the room and attacking them. Eleven confronted the creature, pinning it against the blackboard with her mind. After wishing farewell to her friends, she proceeded to disintegrate the creature, vanishing as well in the process.
One of the theories could be that Will has some supernatural powers that caused the monster to be lured to him even though he wasn't injured. That could also be the reason he could communicate with his mother using the lights.
There is a lot of evidence suggesting both frightening villains were behind the vanishing of Will Byers, but we have to give the win to Vecna. There is much more evidence suggesting that it was the super-powered bad guy, and even if Vecna sent a Demogorgon to kidnap Will, it would still be Vecna who is responsible.
The Duffer Brothers had Vecna planned from the start, which is how they were able to tie his character into earlier seasons. Stranger Things revealed Vecna was also behind the Mind Flayer that appeared in season 2.
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.
"Stranger Things" begins with the disappearance of Will Byers. He was in the Upside Down, far out of reach of any regular human in Hawkins. But when Mike brought Eleven to his house, she saw a photo of Will and recognized him.
Joyce and Chief Hopper found an unconscious Will Byers strung up in the Upside Down version of the public library; he had a tendril in his mouth, extending through his trachea, and into his lungs. Hopper pulled it out of his throat before shooting and destroying it.
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.
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."
After defeating the Demogorgon, Eleven wakes up in the Upside Down. She escapes through a portal which leads her back to Hawkins Middle School. With the government still searching for her, she is forced to hide in the forest. Eleven eventually finds the Eggos Hopper has left for her and seeks him out.
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.
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.
In the end, the Hollands get some closure—if not the total truth—about what happened to their daughter, as Hawkins Lab and the Department of Energy take the blame for Barb's death, pinning it on a made-up “experimental chemical asphyxiant, which had leaked from the grounds of the lab.” (It's a win-win situation, even ...
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.
It was in this moment that Vecna likely created the Upside Down hive mind, becoming the puppet master behind all the monsters while simultaneously growing stronger by absorbing their minds. When Eleven opened the mothergate years later, Vecna was finally able to get a foothold in our world.
The obvious answer: Vecna wasn't written into the show until Season 4. Anything that happened before that season is a result of that.