Joyce, Mike, Lucas, Mike, Robin, Erica, and Murray were trying to wake Will up after their plan to use his connection to Vecna to kill him backfired. Worse yet they released Mind Flayer particles into Hawkins.

Later Joyce was crying at Will’s side as he still hadn’t woken up, she was alone and that’s when the Mind Flayer struck its particles came through the vent like black smoke before Joyce could react her throat, nose, eyes and ears were flooded with the particles, it was like suffocating and once inside, the Mind Flayer began chipping away at Joyce’s control and forcing her to forget it was inside her. The Mind Flayer, planned to use Joyce to sabotage the group’s plans as well as serve as a “spy” since through her new connection to the hive mind, Vecna and the Mind Flayer could see everything she saw

The Mind Flayer whispered in Joyce’s mind constantly trying to seize control.