Chapter 1

Mike didn't know it at first- he thought he just had the flu. But after arriving back in Hawkins after the events of Stranger Things 4 and he learns of the symptoms of Vecna's curse he doesn't think it's a big deal- Vecna had already chosen all his victims to open the gate. But Vecna still has the connection to Mike. Vecna decided to abuse this connection.
AKA
Mike possessed/mind controlled by Vecna
Heavily inspired by a one shot I read on Ao3