The big bad in Stranger Things was her Tulpa which manifested in the real real world.
This happened in real life at Montauk cause people in real life think they have their shit together. They don’t. Not unless they are enlightened and have their egos under complete control.
Everyone’s ego, at it’s worst, is like the beast in Stranger Things.
(yeah, way worse the theoretical “Hitler” argument everyone likes to make)
The season 1 ending could be interpreted as a loose metaphor for her enlightenment.
See this post for further elucidation