Will was riding his bike home when the creature began chasing him. He made it to his house, locked the doors. Then: the front door unlocked by itself using telekinesis. Later information revealed this wasn't the Demogorgon's ability it was Vecna acting through the hive mind. Will ran outside to his shed. Before he could escape, he was pulled into the Upside Down. Will survived for almost a week. He hid in Castle Byers' Upside Down version, his house, the library, and other places while avoiding the Demogorgon. Joyce could sometimes communicate with him through lights because Will was still alive. Eventually the Demogorgon caught him and brought him to Vecna. Vecna didn't want will dead He infected Will with vines and tendrils from the Upside Down, linking Will's mind directly to the hive mind. This made Will a living connection between the two worlds. Even after Joyce and Hopper rescued Will, a tiny piece of the Upside Down remained inside him. The possession Years pass, that connection allowed the Mind Flayer (working with Vecna) to possess Will. Every time Will felt cold or grabbed the back of his neck, it meant Vecna's hive mind was nearby. The Mind Flayer slowly took control of Will's body, spying on his friends through him. Will tried to resist, even tapping Morse code with his fingers to secretly warn everyone that "close gate" was the only way to stop the creature Will can still feel: When Vecna is nearby. What Vecna is feeling. When Vecna is hurt or angry. Activity from the Upside Down. That's why he often touches the back of his neck it has become a warning sign whenever Vecna is close.