I have a theory about this. Firstly, we were not shown what happened to the Spider Monster after season 3. Most likely, his body was examined by the government led by Brenner and Owens. And we should also not forget about the part of the Mind Flayer that remained in the Spider Monster and, as we know, fragments of the Mind Flayer do not die after the gate is closed, but simply fall asleep. Secondly, from Vecna’s prophecy we know about a certain huge creature with an open mouth, but the Mind Flayer in his particle form has no trace of a mouth.And what if the Spider Monster came to life when the gate opened?
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