Warning: SPOILERS for Stranger Things season 4.
How long has Vecna been in the Upside Down and how have those years affected him, turning him into the monster viewers see in Stranger Things season 4? About midway through Stranger Things season 4, Dustin posits that Vecna has been in the Upside Down for decades, seeing as he attacked and killed two members (believed to be three) of the Creel family in 1959 and resurfaced once again in 1986. But how accurate was he?
Crucially, Dustin tells Steve that they don't know how long the Upside Down has existed, because, for all they know, Eleven didn't create the Upside Down when she opened the first gate back in 1983, though that's technically a possibility. It's an alternate dimension stuck on the day that the gate opened and Will Byers disappeared, but there are a number of potential reasons for the Upside Down's timeline. After all, Eleven opened a portal to the world before that didn't set off this chain of events.
Related: Stranger Things' Vecna Identity, Powers & Origin Fully Explained
Overall, Vecna has been in the Upside Down for seven years. In September 1979, Eleven freed Henry Creel/Number One from his metaphorical shackles and inadvertently allowed him to massacre everyone at Hawkins Lab, including all of Dr. Brenner's test subjects. But when she discovered what he had done, she banished him to the Upside Down, creating a temporary portal to send him there, which closed immediately afterward (this was similar to the portal she created when she defeated the Demogorgon at the end of season 1). Therefore, Henry Creel/Vecna has been in the Upside Down for seven years due to season 4 picking up in spring 1986 (so more like 6.5 years).
Vecna wasn't in the Upside Down when
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