1987, Hawkins, Indiana. Who would know that such insignificant town could possess things like other worlds, wormholes, monsters, and, mostly important, kids with superpowers? even though you weren't a child anymore, either your sister, Eleven, wasn't, you two conserved your powers, using them against the Upside Down and all the darkness it caused. But fighting against one with the same abilities isn't gonna end right... Henry Creel. A.K.A Vecna, A.K.A Mr. Whatsit for the kids. But for you, the man who protected you in Hawkins Lab, 001. He was the cause for all the strange things happening in Hawkins; Will being possessed, the kids disappearing, teens dying and even leaving your friend, Max, in a coma. Half of you was pissed, this man was a demon, the pure evilness. But other part knew this wasn't him, Something told you he was a vessel, like Holly, or like Will. But shh, you dare to say that and all of your friends will kill you. Okey?

after multiples missions and calling everyone, you guys idealise a plan for killing Vecna and everything behind him; the Upside Down, the Wormhole, The Abyss and the Mind Flayer. Long story short, you're so far so good! In the phase of 'entering to the Abyss and search for Vecna', you end up fighting with the one and only, Mind Flayer. While you and El try to kill Vecna from inside of it, Mike, Lucas, Dustin, Steve, Robin, Nancy and Jonathan where fighting the demon. The victory finally was claimed by you, and both Henry, and the Mind Flayer were dead. Everyone went and helped all the kids that were inside the meaty place, Derek, Holly, Mary and more. But you felt a strange feeling... guilt? Sadness? For what? Killing Vecna? The monster who created fear to a not-so-innocent town? You're going mad... don't blame you. You felt like you needed to do something, at least say goodbye before making the Upside Down explode and with that, exploding the Abyss too... so you stand there, hand in his wined face, unrecognisable. He was coughing and gasping for air because of his crossed chest. You entered to his mind with your powers, and you saw him, not the monster without mercy, but him, Henry Creel, that man who you once knew. The scene was the same Will had seen when he helped you defeat him. He was watching the worst memory he had, when he was possessed by the Mind Flayer. Henry was trembling, sweating too. The droplets of sweat mixing with the blood ones from an injury across his right cheek. He was... scared...

(srry for the long texttt, I like starting from the start to have more context :D)