Warning: SPOILERS for Stranger Things 4's finale.
Stranger Things season 4 reveals that Henry Creel aka One aka Vecna created the Mind Flayer when he was banished to the Upside Down in 1979. So much has gone unanswered for far too long in Stranger Things. Back in season 1, viewers were given glimpses of the Upside Down, Eleven's powers, and what the Demogorgon was, but that was only the beginning. Soon, the Mind Flayer joined the fray with his Demodogs and mass desiccation across Hawkins. Now, answers have finally arrived in Stranger Things season 4.
Leading into season 4's release, the Duffer Brothers noted several times that this would be the season of answers and reveals - and they weren't kidding. Volume 1 explained various aspects of the Upside Down, Eleven's backstory, and even character motivations that have otherwise been enigmatic for the past few seasons. But Volume 2 provides even more details, particularly about Vecna and the Mind Flayer. Earlier in the season, Dustin postulated Vecna was the Mind Flayer's five-star general, his second-in-command. That couldn't have been further from the truth.
As it turns out, Vecna hasn't been doing the Mind Flayer's bidding; it's been the opposite all along. Vecna created the Mind Flayer when he arrived in the Upside Down and identified the Mind Flayer's particles/hive mind as being the missing piece in his plan to gain ultimate power. Whenever Vecna kills someone, he "consumes" them, according to Dr. Brenner; that means he acquires their consciousness and power. By using his own powers, Henry/One took control of the Mind Flayer and obtained all of his powers, thus becoming stronger and in control of the Upside Down.
What Powers The Mind Flayer Gave Vecna
Even before Henry Creel became One, he had powers that enabled him to see into other people's minds and kill them by using their worst memories against them. It's what eventually drew Dr. Brenner to him, once Henry had killed his mother and sister. But his powers could only go so far. While his telepathy and telekinesis were useful, absorbing the Mind Flayer particles in the Upside Down (or whatever the version of the Upside Down was before 1983) connected him to this new world. In his own words, he finally broke free of his metaphorical bonds and became the predator.
Connecting to the Mind Flayer's hive mind allowed Henry Creel to become the version of Vecna that audiences see in Stranger Things season 4, controlling the Demobats, Demodogs, and Demogorgons across the Upside Down and the real world.
Why The Mind Flayer Looks Like A Spider
Interestingly, Henry Creel never gave up on his fascination with spiders. Right down to the end, Henry took control of the Mind Flayer particles and gave it form, turning the shadow monster into the Mind Flayer, a spider-like creature. It was based on his own design, which he sketched when he was still a kid living at the Creel house. Thematically, turning the Mind Flayer into a spider-like creature was keeping in with the monster's abilities.
The spiders in the Creel house were primarily black widows; they have a venomous bite that results in pain similar to what Will Byers felt when he was attacked. Furthermore, black widow bite symptoms typically last for about seven days, which is the same length as Vecna's curse. While that's separate from what the Mind Flayer was doing to Will in season 2, the idea of the monster attacking, biting, and infecting its prey tracks with what happened in season 3.
Why Vecna Used The Mind Flayer In Stranger Things Seasons 2 & 3
The big question is, if Vecna was responsible for everything all this time, why didn't he attack Hawkins earlier? At first, the Demogorgon in Stranger Things season 1 and the Mind Flayer in Stranger Things season 2 were means to an end, to occupy Hawkins, but they were ultimately stopped by Eleven. She killed the Demogorgon and shut the gate on the Mind Flayer, so in order to circumvent Eleven's resistance, Vecna built the Mind Flayer in the real world and siphoned Eleven's powers when it bit her. That bite presumably was partially responsible for blocking Eleven's powers at the end of season 3 and the beginning of season 4.
But once Vecna acquired Eleven's powers, at least a part of it, he was able to access people's minds across dimensions, killing them from the Upside Down and using their deaths as a means to open new gates. None of that would've been possible if it wasn't for Eleven. After the Mind Flayer failed in its original purpose, it served a new one. However, the intriguing thing is that since the Mind Flayer was removed from Will Byers, it left an imprint on him, thus meaning Will is still connected to Vecna, even if the monster doesn't know it.