Now that season 4 volume 2 has been released on Netflix, we can talk about the major revelations made throughout the two episodes. One of them was that Will is somehow connected to Vecna.
The connection between Will and the Upside Down was established way back in season 1 when he got stuck in the parallel world. Even though he was rescued but he got possessed by Mind Flayer in season 2. Once again he managed to survive thanks to his friends and family who helped him to banish the Mind Flayer. But it seems a residual psychic connection has remained.
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In season 4, it looked like that Eleven has defeated Vecna and he is dead. But soon we got to know that this is not the case. When Will comes back to Hawkins, he finds the weakening of the dimensional barriers means he can now sense Vecna's thoughts. He tells Mike that Vecna is still alive and he can feel him.
Now it will be interesting to see how this plays out in Stranger Things season 5. Will can help to defeat Vecna or maybe he will become Vecna's own unwilling agent as there have been many theories about Will Byers turning evil.
If we observe keenly, there are similarities between Harry Potter and Stranger Things. There, Harry Potter and Voldemort were bound together because of Voldemort's Killing Curse. In the same way, Will and Vecna appear to be connected because of Vecna's short-sighted plans in earlier seasons. Thus Will who was the victim in season 1, could become the one who ultimately defeats Vecna.
He's not, he's connected to the UpsideDown