I’m not saying it’s Mephisto but I think the Mephisto theory can be attributed to her admitting that she was in a hopeless dark place, with the body of a very dead version of Vision and no memory on how she got to where she is now. Who or what else comes to you when you have nowhere to turn and you are at your lowest? (in fictional movies). It can be a number of people or entities.
I like the idea of the mind stone possessing or influencing Wanda but the mind stone was already possessing people in the first avengers, it’s been done.
Also Wanda being a villain/hero is repetitive as she started out as a villain in AOU and then became a hero. This feels like a classic case of someone taking advantage of another person’s grief.
Instead of her becoming the villain in the Strange Movie, he might take her under his wing so that she can learn to control her powers. This way she could fight and ward off any supernatural entities going forward
I keep thinking about how Visions coworker was going through grief of his fathers illness when he regained control and I don’t think that was a coincidence either. The hex probably feeds on negative emotions.
My theory is that Wanda got visions body and Mephisto approached her and told her the only way to bring him back was to recreate the stones (or take some from someplace else) and she created the Hex. The effects of recreating the infinity stones (aka the commercials) is also opening the Multiverse and Mephisto not being able to bring someone dead back to life instead grabbed a QS from another universe to keep her occupied and filled his head up with the prior MCU QS’s memories and he is controlling him. I wouldn’t be surprised if we find out that physically Mephisto can’t interact on earth or something. Which could be cool, because he could be played by a lot of actors/actresses in other shows.
Mephisto needs to kids to be ripe (aka older) to then take the stones and subconsciously Wanda is trying to hide the stones from him. Also Agness is definitely working with Mephisto and keeps things moving along as planned.
I actually think the kids are the stones. There is a good chance both Vision and the kids do not survive this.......