The slug
Will Byers: Will was found in the Upside Down with a tendril extending down his throat following his capture by the original Demogorgon (see Stage Six below). A month after returning home, Will coughed up a larva and washed it down his bathroom sink drain.
Hopper had to yank that thing out of Will's throat and then provide some (questionably rough) CPR to get him breathing again. But in the season's closing moments, after a very merry Christmas Eve dinner with Joyce and Jonathan, we saw Will run off to the bathroom to cough up a slimy green slug.
Being an adolescent Demogorgon, Dart possessed abilities that are characteristic of his species: Strength: After molting to his Demodog form, Dart possessed enough strength to tackle an adult human. Durability: As a Demodog, Dart's skin was thick enough to withstand several gunshots.
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.
Will said himself that he remembers seeing Dart before, only when it came out of his mouth at the end of Season 1, it didn't have a tail. It seems to have grown and evolved since Will last saw it, but since it came out of Will and he was in the Upside Down for so long, that seems to be the root of Dart's origins.
The author of the theory explains that the scientist that was attacked by the monster at Hawkins Lab at the very beginning of the series wasn't bleeding, but he was running.
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.
Thankfully, Dustin was able to recognize him by the yellow stripe on his back, and Dart remembered Dustin as well.
Before Stranger Things introduced viewers to eldritch horrors like the Mind Flayer or bogeymen like Vecna, the primary creature menacing Hawkins was the Demogorgon. Like all the show's creatures, it was inspired by an iconic horror movie -- in this case, Steven Spielberg's 1975 classic Jaws.
Once a happily married detective living in New York, Hopper lost his daughter Sara to cancer at a young age. His marriage to Sara's mother Diane crumbled soon after, prompting Hopper to move back to his hometown in Hawkins, Indiana.
In 1986, Will started to act more mature and even said a few curse words like his friends normally do.
After refusing One's invitation to destroy the world, Eleven used the power of her happy memories and defeated One, pushing him into the Upside Down where he became Vecna.
In Stranger Things season 1, Will is attacked and captured by the Demogorgon. His vanishing kicks off the plot of the whole show and he survives for a long, lonely week in the mysterious Upside-Down after being snatched by the toothy-mawed monster.
When Will was trapped in the Upside Down and captured by a Demogorgon, the Mind Flayer became interested in Will and decided to use him for his own purposes, showing no care for him.
Susan: Will is the spy into the Shadow Monster's motives, but it appears like he's a double agent. As his hippocampus abnormalities grow, his own memories of people are being replaced by not only the “now memories” but he's a slave to the evil's bidding.
With good reason, Episode 3 then gets named after what they call the creature: “The Pollywog.” Desperate to have something of his own, Dustin adopts the weird creature, naming it D'Artagnan, or Dart for short.
Well, it turns out they're the same creature, just at different stages of growth. Yes, you read that right–a Demodog is just a teenage Demogorgon. The main difference between a Demodog and a Demogorgon is simply that the Demodog is younger and not fully grown.
In the third episode of Stranger Things season 2 “The Pollywog,” Dustin makes a slimy new frenemy who he names “D'artagnan” — Dart for short — after The Three Musketeers character. Dustin is able to satisfy Dart's cravings with his favorite candy bar, 3 Musketeers. That is, until Dart gets a bigger appetite.
At his empire's height, Vecna was betrayed and destroyed by his most trusted lieutenant, a vampire called Kas the Bloody-Handed, using a magical sword that Vecna himself had crafted for him, now known as the Sword of Kas.
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.
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).
After defeating the Demogorgon (which may have been acting on Henry's orders), Eleven vanished inside the Upside Down and then escaped — eventually going to live with Hopper in a secret cabin in the woods.
The creature was finally defeated in a showdown with Eleven at Hawkins Middle School. Eleven used her abilities to disintegrate the creature, but accidentally transported herself to the Upside Down in the process.
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.