Warning: This article contains spoilers for Stranger Things season 4, volume 2.
A lingering question about Stranger Things season 4's villain, Vecna, was hinted at in the season finale. Vecna's true identity was revealed in Stranger Things season 4, volume 1 to be Henry Creel or One, a powerful psychic with a number of terrifying abilities. Still, before Stranger Things season 4, it didn't appear that Vecna was able to open portals to the Upside Down.
Stranger Things season 4 revealed Vecna's plan to force the Upside Down into Hawkins in a big way by using the psychic link created by murdering disturbed teenagers to open enough small portals to cause a sort of tear between the two dimensions. The apparent number needed to make his plans happen is four murders and four portals. By the end of Stranger Things season 4, he's done exactly this and the season ends with the Upside Down bleeding into Hawkins and the Hawkins crew looking on in horror and terror.
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In earlier seasons, Vecna relied completely upon existing portals to act in Hawkins. Vecna didn't attack Hawkins teens in earlier seasons of Stranger Things because he couldn't — he had no ability to open his own portals. The only person shown to have this ability in the series is Eleven and the reveal that One takes the power of anything he consumes is key to understanding his plan in seasons 3 and 4 of Stranger Things. After Eleven closed the big portal to the Upside Down in season 2, Vecna only had a slight lingering connection to the real world through the Mind Flayer. He employed this connection to get close enough to Eleven to get a piece of her, something he was finally able to achieve in season
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