Issue #134
In order to create a film with successful, yet subtly easter eggs, you have to know a lot about different cinematic masterpieces. And it is safe to say that along with the other writers, the Duffer brothers definitely did in-depth research in order to make such great and indirect references.
The #134 issue of X-Men released in 1980, is the first time that readers are introduced to the Dark Phoenix, Jean Grey’s persona. Her powerful alter-ego definitely caused a lot of trouble for the X-Men, a path which Eleven chose not to go down. Similar to how Phoenix got rid of Mastermind, Eleven flattens the Demogorgon against a wall in the lab.