It's not just a mirror, it is a mirror frozen of the very day Will disappeared. El didn't send Henry off on that day, so it has to tie into Will or something that happened on that day. Only thing I can think of is they played their game of D&D on that day, and didn't finish it; like Lucas said, if Mike doesn't know it was a 7 it doesn't count, but Will went and told him. I think it ties into that.
I'm ever more convinced the kids will play a game of D&D in the UD to win. Erica now being a player and such is going to tie into this,and in a way can be part of Eddie's legacy.
I don't remember and can't check myself, do they specify that UD Hawkins is set on THE DAY that Will went missing or just that it's stuck in 1983?It could be that the upside down is influenced by strong desires. So in Will's case, his desire to go home some how created a mirror image of Hawkins.
Also Vecna entered the UD in 1979, btw.
Except that Henry didn't create anything, the first thing he sees in the UD when he gets there is a Demodog walking around on the rocks. Also the flesh monster isn't a unique separate entity it's the Mind Flayer taking physical form in their reality.So the Mindflayer is basically the "power" or "essence" of a Dungeon Master, right? Makes sense. It controls everything in the Upside Down. Henry comes and "corrupts it" giving him control of it which allows him to make his own "campaign". He then creates a campaign to invade earth: demodogs, demobats, flesh monster, etc, and allows him to see through (knowing her friends would die from the vines, demodog, demogorgon, demobats) similar to how a Dungeon Master can "see" into monsters by knowing what their stats are and when they are engaged by the players. This makes sense because he can now bend what the Upside Down is whether it be Hawkins or create things. The Upsidedown is basically a blank slate for what you want to make your campaign on.
Seriously, the finale of the show isn't gonna be the kids playing D&D against a monster and their game isn't coming to life with the series just being their imaginations with invented friends as they playPeople are reading way too hard into the D&D allegory.
It's just a way for a bunch of kids to make sense of the nightmarish shit that's going on around them.
And resetting back to the beginning so they can finish their game is some Dark Tower fucking trash bullshit ending. Absolutely not.