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.
Related: Of Course Eddie's Upside Down Song Was Perfect!
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.
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