There are many magic users from Marvel Comics who would be capable of giving Wanda exactly what she wants in exchange for her help in some nefarious plan, but there are two most likely candidates: the witch Agatha Harkness and the devil, Mephisto. Both characters have a history with Scarlet Witch in the comics, and WandaVision has hinted at both characters being involved with Westview.
Agatha Harkness is a witch from the comics who interacts with several Marvel characters, including the Fantastic Four and Scarlet Witch. Not strictly an antagonist, Agatha was a tutor of Scarlet Witch’s for a time, but when Scarlet Witch lost her children (due to them being unknowingly created from Mephisto’s soul and then reabsorbed upon his return), Agatha wiped her mind of any memory of them. When Scarlet Witch learned the truth of what Agatha did, she killed her, went insane with grief and bent reality to her will. On WandaVision, Agnes the neighbor is suspected of being the MCU’s version of Agatha. Unlike the other residents of Westview, Agnes doesn’t appear to be as fully under Wanda’s control, and this could mean she’s aware of what’s actually going on in. She may have even had a hand in creating Westview, and is now manipulating Wanda to keep up the charade.
The other and frankly more likely culprit for who is really responsible for Westview is Mephisto. The powerful demon is known for making deals with characters when they’re at their lowest, making Wanda a prime candidate. He’s also very keen on collecting and trapping souls, so a scenario in where some 3,000 people are trapped within a town would certainly be appealing. Mephisto’s deals usually work by him fulfilling some desire in exchange for a service. In the case of WandaVision, Mephisto may have offered Wanda what she wants most of all – a happy life with Vision – in exchange for her creating the Westview anomaly and trapping an entire town. As a powerful, immortal being, Mephisto could easily perform the kind of magic even Wanda cannot: reforming Vision’s body and resurrecting him.
Originally from https://screenrant.com/wandavision-scarlet-witch-secret-villain-identity-evidence/