Dimension X

• Vecna did NOT make the Mind Flayer; he simply shaped it.

• Vecna and the Mind Flayer are SEPARATE entities.

• It is unclear which one controls the other; the show has led us to believe that Vecna controls the Mind Flayer, while the added context from the play (which is 100% canon, as confirmed by season 5) explicitly says that the Mind Flayer has more control over Vecna.

• The canyons in Camazotz, where Vecna is scared to enter, were the original place where he got transported to Dimension X and his blood got altered by the Mind Flayer particles. This is from the play.

The Upside Down is a snapshot of November 6, 1983. It was created exactly when Will went missing. When Henry fell to Dimension X, there was no Upside Down. The zone with the lightning he fell through was the Hellscape, a bordering zone. There were no gates between either side besides the one 1979 El deliberately opened. The Upside Down formed in the Hellscape when El reached the Demogorgon in 1983.

Notes:

• When El reached the Demogorgon in Dimension X, the telepathy collided the branes (dimensional planes) of Dimension X and the Right Side Up. This created an aberration in the Hellscape. The Upside Down was formed. This is why there's Spores and Vines from Dimension X, but a copy of Hawkins.

• The Upside Down's creation on November 6, 1983 made the accessibility between the Right Side Up and Dimension X possible by that point, and not deadly. This is exactly the same day when Will went missing, and Henry was able to cross dimensions. El opened the gates between all three zones, making it easy for Henry all along to start his murderous trial and his reign of terror capturing Will (as shown in the first episode of Season 5). In a word, Will was Vecna's first "victim", and Vecna used that Demogorgon as a proxy. If not for that telepathic remark with the Demogorgon, Henry would have stayed in Dimension X.

• Yes, I am saying El created the Upside Down. We're not even sure if the Upside Down is just a copy of Hawkins, which seems likely due to the failed Russian experiment.

I waited until the first 5 minutes came out before posting this, but here goes...

Theory 1: Demogorgons & Project MKUltra

I believe that the first generation of Demogorgons were members of Project MKUltra. This is mostly due to the fact that Demogorgon bodies are too coincidentally similar to the human anatomy. I think it was certainly a failed project by Brenner, especially to silence the testees. I think Terry was excluded because she had obviously already been very vocal about the project since even before they could do anything to her. This also supports the popular theory that Dr. Brenner knew about Dimension X well before the 1979 incident between 011 and 001.

Theory 2: Upside Down & the future

Now, this is a very popular theory, but I wanted to expand it a bit. The basic theory is that the Upside Down is what will happen to Hawkins/the world in the future. My theory expands to the idea that Dimension X is an even further future state of the planet as a whole, and whatever happened in 1983 caused a paradox that created a dimension which replicates the earlier future of what the world would become, but limited to Hawkins for a reason to be told. Now this theory dictates that Season 5's plot will not only be killing Vecna, but also stopping Vecna from causing the very event that will lead to the Earth's state becoming that of the Upside Down and, eventually, Dimension X. So, essentially, Vecna would be trying to cause an incident that leads to a future which is also the past that led to his creation & power.

What prompted me to this theory was that I relatively recently watched the TV series Lost. Anyone who has also watched the series cannot help but admit that the ST and Lost universes are somewhat similar.

So I think that the Upside Down is possibly hell, because outwardly hell and UD are similar (red sky, demons and the devil). The Devil is a Mind Eater because in season 2 Will literally tells Dr. Owens that he is Evil. Therefore, MF is possibly the embodiment of evil. See for yourself: he seems to consist of darkness, his goals are to create chaos and kill all living things, he can also “corrupt” people by inhabiting them and granting strength and almost immortality, and control over demogorgons. Just a real Devil!

And Dimension X is purgatory because there, as you can see, the monsters behave calmly, take Vecna ​​for example. He walked around the Dimension for many years and was not attacked by either demogorgons or MF. In addition, Dimension X looks calm: soft yellow skies, smooth rocks, flying stones and livitizing particles.

As we know at the moment, we have three theories about who created the Upside Down (not Dimension X). But I have another theory that the Upside Down was created not by Eleven, not by Henry and not by Will, but by the Mind Eater.

I came up with this theory after analyzing the trailer for the play. If you noticed, the play showed more than once how the Mind Eater takes Henry to the Upside Down, and not mentally but physically. And not only Henry himself, but also other people like Petty and her father. You can tell me that I misunderstood and the Mind Flayer could not physically transfer other people. Then watch the scene where Henry almost killed Petty and her father, and in this scene all three were enveloped in vines. Please note that this is not Henry's mind, because Henry's mind looks different, it is the Upside Down.

Here are some arguments that this is not Dimension X, but the Upside Down.

In Dimension X, the sky is yellow, there are stones and particles of the Mind Eater flying everywhere. In the Upside Down, the sky is characteristically red and it reflects our world and the Mind Tormentor transported Henry to a place exactly in a place that copies ours (it copied Henry’s house) and with a red sky. Moreover, I do not deny that the Upside Down is stuck in 1983. Henry's house in the scene where he is thrown into the Upside Down looks old and destroyed, and at that time in the Upside Down it was already 1983.